Throw It In/Film

"Rick: Here's lookin' at you, kid."
 * A much-disputed, yet nonetheless famous instance, is in Midnight Cowboy. One of the producers insists that the cab that prompted Hoffman's now-famous "I'm walkin' here, I'm walkin' here!" was driven by an actor, and that the production team was told to make the near-hit appear to be ad-libbed. However, when on Inside The Actors Studio, Hoffman claimed that he and Voight were not supposed to be nearly hit by any traffic, even from paid drivers, and that his reaction was in lieu of "We're filming a movie here!"
 * A few John Belushi moments in Animal House came about like this, particularly in the cafeteria scene. His trip through the buffet line was between takes, but when the crew saw they were told to keep rolling. Moments later, he improvised the "I'm a zit" gag, and the looks of surprise and disgust on the actors are genuine.
 * In the 1967 film, The Dirty Dozen. Lee Marvin's "Oh, they played an active part alright." line was completely unplanned, as was Ernest Borgnine's reaction of spitting his drink on the floor and coughing.
 * In 28 Days Gerhardt's speech about forks in the road, salad forks, crab forks and ladles was entirely ad-libbed by Alan Tudyk.
 * Titanic
 * Leonardo DiCaprio telling Kate Winslet to get on the daybed in preparation for him sketching her nude portrait, saying "Get on the bed -- errr, couch!" According to the director's commentary, the original line had no reference to a bed, but DiCaprio's nervous flubbing of the line seemed just too perfect to leave out.
 * When the ship is sinking and Rose comes to save Jack, when he jumps in the rising water, he says, "Shit, that's cold!" -- apparently, unscripted. Rose goes "GAAAAASP!" in the same scene. This was also Enforced Method Acting, as the actors were told the tank of water would be warmer than it was.
 * When Jack and Rose are hanging on for dear life about two minutes before the ship sinks, Rose says "Jack, this is where we first met!". Complete ad-lib, but it makes the scene that much sadder.
 * Also an ad-lib was Jack's line as he is leaving the First Class dinner table: "Time for me to go back and row with the other slaves." James Cameron preferred it to the scripted line and left it in.
 * Robin Williams was also notorious for ad libbing a large part of his dialog -- it's said that often the writers ended up saying, "Well, that's funnier, let's go with it" to his improvisation on-stage.
 * Good Morning Vietnam, What Dreams May Come, and Patch Adams all feature examples of Williams ad-libbing.
 * Mrs. Doubtfire has both in-character and out-of-character examples:
 * The movie starts with his character, a professional voice actor, quitting his job because they won't allow him to comment on the cartoon's message that if someone offers you a cigarette, you should take it.
 * Actor Robert Prosky described his approach for the restaurant scene in Mrs. Doubtfire as "hold on for dear life" since he never knew exactly what was going to come out of Williams' mouth during any given take. If you watch that scene carefully, you can see Pierce Brosnan struggling not to crack up at Robin's antics, and this is made all the funnier by the fact that Brosnan's character is supposed to be annoyed/angry throughout most of that scene.
 * Much of the monologue in Good Will Hunting where Robin Williams' character is counseling Matt Damon on relationships was ad-libbed. This is particularly true in a bit where Williams is describing his dead wife and her tendency to be flatulent when sleeping, which is why Will responds by laughing almost hysterically -- Matt Damon himself had no idea what was coming. You can also see the camera shaking very slightly, and it's been reported that the cameraman too was laughing. His last words ("Son of a bitch, he stole my line") were also improvised. So was "Fuck you!" "You're the shepherd."
 * During filming of The Birdcage, Robin Williams and Nathan Lane were so thoroughly into ad-libbing and bouncing off one another that they were forced to promise they'd do one take exactly as scripted before they were allowed, in subsequent takes, to say whatever they wanted. Also, the scene where Robin Williams trips carrying the pot of soup was not supposed to happen, but how hilariously appropriate it was to the mood made it into the film. If you pay enough attention, Hank Azaria nearly loses it at Robin falling.
 * Pirates of the Caribbean and its sequels.
 * In Curse of the Black Pearl, Jack's statement that he used "human hair -- from my back" was an ad-lib. You can see Bloom trying not to laugh, and McNally chuckling in the movie. The commentary states that they initially tried to edit it out, but they found that the line lost something without it, so they threw it in.
 * In the trailer for Pirates of the Caribbean: Dead Man's Chest, Will says to the other characters "I'm not leaving without Jack!" while getting on the Black Pearl. When he sees that Jack is on the other side of the beach getting chased by a large group of natives, he says, "Never mind, let's go!" This line didn't make it into the movie (because they did indeed wait for Jack, more or less), but its creation was actually from a blooper where Bloom flubbed his line, and "Never mind, let's go!" was an effort to just keep going and say the line again without hesitation. This fact can be found on the Dead Man's Chest DVD commentary.
 * All of Jack's jokes about Will supposedly being a eunuch were ad-libbed by Johnny Depp. Through the creators' approval of the first, he continued.
 * According to sources, pretty much the whole of the character in the first film was a Throw It In, as originally, Jack was supposed to be more of a background character, a sort of sidekick to William, but Depp decided to go with his now famous zany approach to the character, that for a while, had the director and the executives not too pleased with him. Depp told them "trust me, or fire me." He still improvised a lot in the second and third films, but in the first film, it led to a completely different character.
 * Speaking of live-action Disney films set at sea, in Twenty Thousand Leagues Under the Sea, Kirk Douglas falling over in his haste to row to safety was an accident too funny to pass over.
 * District 9: Many, if not all of Wikus' lines are improvised. When you consider how beautifully Sharlto Copely acts his part, this becomes really impressive food for thought.
 * It's a Wonderful Life examples:
 * Thomas Mitchell, the actor playing the drunken uncle is accompanied by a loud crash on one of his exits; the noise was actually caused by a grip tripping over a prop table and scattering its contents, but the timing was so serendipitous that director Frank Capra decided to use the take anyway. They were going to re-take it, but Mitchell shouted "I'm alright! I'm aallllll.. .right." That saved the take, as it made it look like he'd just done an off-screen collision with a garbage can. The grip thought he would be fired on the spot. Instead, Capra gave him a $10 bonus for "improving the audio quality of the movie."
 * There was much more dialogue in the scene where George and Mary are both talking to Sam over the phone -- but that long kiss was so much better than the dialogue Capra scripted that it got used instead. Technically, that might be "Throw It Out" as much as "Throw It In".
 * In the building and loan panic scene, the woman asking for $17.50 wasn't originally in the script. Capra fed the actress the line before shooting without telling James Stewart, so he could seem genuinely surprised when she said it. He was, and the grateful kiss afterward was an in-character ad-lib on Stewart's part.
 * Guardians of the Galaxy; In the scene where Starlord first meets the Collector, he is about to give him the orb - not knowing it contains an Infinity Gem - and drops, it picking it up again as if nothing had happened. Given his personality, most fans assumed this was in the strip, but it was not. This was actually a mistake made by Pratt which director James Gunn loved so much, he kept it in.
 * The Lord of the Rings
 * In The Fellowship of the Ring, Ian McKellen accidentally hit his head on the ceiling while entering Bilbo's residence. This was kept in the final cut as a joke. Bumping the hanging lanterns was scripted. His quick turn to his left, apparently to avoid the hanging lanterns... results in a painful whack into the low ceiling-strut as he does so, and this was not scripted. Ow...
 * Also in the first film, during the final fight between Aragorn and the leader of the Uruk-Hai hunting party, the actor in the Uruk makeup was supposed to fake a head-butt to Viggo Mortensen. But the makeup evidently made it difficult for him to judge the distance, and ended up giving Mortensen a very real head-butt. The move, and Mortensen's very real pain, made it into the film.
 * From the same fight scene, Makaore (the actor playing the Uruk) throws a knife at Mortensen. The script called for him to throw it and miss, but he actually threw it straight at Mortensen... and Mortensen deflected it with his sword, a completely unplanned move so cool that it would be hard to believe it wasn't either practiced, or special effects.
 * In The Two Towers, Aragorn comes upon a scene that seems to indicate that two of the other characters are dead. On the extended DVD, they show several takes of him snarling in helpless anger as he kicks an Orcish helmet. Then, they try one more take... and he collapses to his knees, screaming in fury, grief -- and pain. Viggo Mortensen broke two toes kicking the helmet and decided to use it. They finished the shot, and then brought the medics in. Cracked.com's 6 Iconic Movie Scenes That Happened by Accident cites this broken toe among other iconic accidents.
 * In the scene where Eowyn runs onto the terrace after her confrontation with Wormtongue and stares out at the landscape, a banner suddenly tears off its pole and blows away. That was not intended; genuine high winds were blowing everything around violently. But the symbolism was so provocative and moving it was decided to keep the shot, and a follow-up scene was later filmed showing the banner landing near Aragorn as he rides up to the base of the hill.
 * During filming of the Battle of Helms Deep, some of the stuntmen playing Uruk-Hai relieved their boredom between takes by making a game of tapping their spears against the ground in unison. This gave Peter Jackson the idea to have all 10,000 Uruk-Hai do it as they arrived as an intimidation method.
 * One specific instance is described on the special edition extra content, where they were about to shoot the Elves counter-attacking against the Uruks coming through the breach in the deeping wall. The actors playing the Elves were a little timid (probably their first shoot as Elvish warriors). At the time, there was a large group of stuntmen in Uruk costumes standing across the way, who began stomping their feet, beating their weapons against their chests, calling them names and even making obscene gestures at them. This quickly got the Elvish stunt-doubles riled up, and they in turn began posturing and drawing imaginary arrows at the Uruks. Then suddenly, to everyone's surprise, the director yelled "Cut!". Part of the footage got into the film, though they had to cut out parts with gestures and exclamations that were not... native to Middle Earth.
 * In First Blood, protagonist John Rambo jumps off a cliff into a tree, then falls down, hitting branches on the way down, to hit the ground with a blood-curdling scream. That's because Stallone broke three ribs doing the stunt.
 * Similarly, in The Young Lions, when Marlon Brando's character, he falls down a big hill and into a pond. He apparently injured himself rather badly in the fall, but being the world's most famous Method actor, he kept still and finished the take and waited to yell in pain until "cut" was called.
 * Silence of the Lambs
 * Hannibal Lecter's famous hissing was completely improvised; indeed, was enough of a joke that the actors didn't expect it to be kept in the film. You'll notice that there's a nice long pause between "A nice Chianti" and the hiss, presumably so that it could be cut without damaging the line. The director decided it struck the right tone, after noticing Jodie Foster was quite genuinely creeped out.
 * Hopkins improvised the bit where he briefly mocks Clarice's accent during his Hannibal Lecture; this overlaps with Enforced Method Acting because he did not inform Foster that he was going to do this, so the surprise on her face is genuine.
 * Several scenes in The Film of the Series of Bewitched were directly scripted from development-period improvisations between Nicole Kidman and Will Ferrell, including, in an amusing recursion, the scene where Isabel and Jack "improvise" an interview with a witch. According to Nora Ephron's DVD Commentary, this scene was essentially unchanged from the original improv, right down to Kidman's line, "Do we have to keep doing this?"
 * X-Men 1: Reports suggest that the actual script of the movie didn't have Wolverine saying "bub" but Hugh Jackman, as a fan of the character, threw it in. Some reports state that he actually "threw it in" many, many times. They just only kept some of them. When Wolverine meet Professor X, he says "What do they call you -- Wheels?" where the Wheels part was ad-libbed. The scripted line was "What do they call you -- Baldie?"
 * After the famous lobby fight scene in The Matrix, there is one final shot of the collateral damage in the lobby; a piece of one of the pillars collapses, which hadn't been originally intended, but it looked cool, so they kept it.
 * There's also the part where Neo vomits after being told my Morpheus the truth of the Matrix. In the set, Keanu really vomited after something in his meal didn't agree with him.
 * According to the director, much of the humor in Death at a Funeral was based on deliberately exploiting this trope. He explained that scenes would often be repeated until something funny went wrong, and then that take was used.
 * The 40-Year-Old Virgin
 * The "You know how I know you're gay?" scene sprang from an improvisation about a completely different subject.
 * The waxing scene: they didn't tell Steve Carell on the first rip that they were actually going to go through with it. Hence his expression, followed by some decidedly out-of-character swearing at the actor who just ripped half his chest hair off. Those wincing looks and glances off-camera from his "buddies" are real. Carell also ad-libbed all the lines he yelled after each rip including "Kelly Clarkson!!" The script for this scene actually read: "Scream, swear, apologize," if memory serves.
 * Marx Brothers
 * Groucho Marx ad-libbed frequently; many Marx Brothers movies have noticeable blips where the makers shaved off a few seconds to make room for things like the Animal Crackers speech which begins: "Pardon me for a second while I have a strange interlude."
 * Some scripts simply had "Harpo Does Something Funny" because his improvisations were often better than what the writers could come up with.
 * Their first major film The Cocoanuts had to be shot with multiple cameras because every take they did was different, so normal single camera techniques didn't work. The "viaduct" gag was not in the original script of the play the film was based on.
 * In Animal Crackers the actor playing the antagonist accidentally called Groucho by his own character's name. Both of them were able to improvise off it well enough that the take ended up in the film.
 * More of a funny mistake than an intentional improvisation, the film Hot Fuzz includes a scene where Simon Skinner, Timothy Dalton's intentionally-played up bad guy  raises his glass and for a split second looks right down the barrel of the camera. Director Edgar Wright decided to leave the outtake in, and even timed the sound of a bell in the background to accompany it. Additionally, Danny Butterman's "I'm not made of eyes" was ad-libbed by the actor. Similarly, the first scene where Dalton's character gets introduced (when the two are jogging) was going to be reshot, because Dalton kept unintentionally pushing Pegg out of frame. They decided to keep it in, as they felt it fit Skinner's character.
 * The most famous example of all time, in Casablanca:

""Don Corleone, I am honored and grateful that you have invited me to your daughter...'s wedding... on the day of your daughter's wedding. And I hope their first child be a masculine child. I pledge my ever-ending loyalty.""
 * Borat: the naked wrestling scene. Sacha Baron Cohen told the director that if he ran short on oxygen from having a 300 lb man sit on his chest he'd hit the mattress three times fast. If you look you'll find he does that about halfway through the fight.
 * Annie Hall: the scene where Alvy sneezes, blowing away a boxful of his friend's $2000/oz cocaine ran much longer, but was cut back because the laughter from the audience made the rest of the dialog inaudible. The sneeze was real, and unrehearsed.
 * Aliens: "Game over, man! Game over!"
 * A example that has become a legendary scene: Indiana Jones shooting the swordsman in Raiders of the Lost Ark. Harrison Ford had diarrhea problems and wasn't up to fight him with his whip as originally scripted. A version of the story exist that Ford improvised the scene while filming. A slightly more plausible version says that Steven Spielberg said sarcastically to Ford that the only way the scene could get shortened is if he just shot the guy. The crew began laughing at the idea and they worked it out. Another version of this story holds that Ford and most of the crew had gotten sick, and this was the last scene they needed to film in this location. Ford goes to Spielberg and says "Look, Indy wants to save the girl, right? He doesn't have time for this, so why not have Indy shoot the fucker?" And so he did. According to the Making of Indiana Jones book, they did manage to shoot a completed fight scene with the swordsman. There were two versions during the editing process. One cut with the fight with the Arab and one with Indy just shooting him, with George Lucas preferring the former and Spielberg the latter. They left it to a test screening to decide which to use. Indy shooting got the biggest laugh and was kept in.
 * In Indiana Jones and the Last Crusade, in response to Indy asking his father how he knew Elsa was a Nazi, Jones Sr. simply replies "She talks in her sleep." Sean Connery actually ad-libbed that line, and it was kept since it made the entire crew burst into laughter.
 * Star Wars series
 * The Empire Strikes Back: Han's reply to Princess Leia saying that she loves him was originally supposed to be "I love you too," but Ford ad-libbed "I know," because he felt it to be more like the character. The director Irvin Kershner said they had ran through several different lines because the "I love you too" line felt too lovey-dovey for someone like the Loveable Rogue Han Solo, so eventually he just asked Ford to say what felt natural. Kershner loved the result -- "I know" was the final take, at least before lunch -- but George Lucas was afraid it was Mood Whiplash. Lucas was proven right when airing for a test audience -- but the audience also felt the line was classic Han Solo, so he agreed to leave it in.
 * In A New Hope when Luke and Han were rescuing Leia disguised as stormtroopers and Han was forced to respond via radio to their commander, Ford intentionally did not memorize his lines and only briefly looked at what he was supposed to say. So while the scene as written is supposed to be Han improvising, "We're all fine here, thank you... How are you?" Ford played it panicked and grimacing at the last line. Again, it was good enough to keep, and provides a great bit of comic relief in the middle of a tense sequence. Harrison Ford just seems to be a magnet for these.
 * He pulled a similar stunt while filming The Fugitive (see details below).
 * Apparently Luke's remark "I can't see a thing in this helmet!" regarding his Stormtrooper disguise, was made by Mark Hamill after he thought the cameras had stopped rolling. This led to another Throw It In moment, when the Stormtrooper smacks his head on the door. A moment that's so famous, recent DVD releases apparently add a "thud" sound effect when it happens. You just can't get the help these days, can you Lord Vader?
 * Jango Fett gets banged slightly by a descending ship door in Attack of the Clones in homage of the above, even though that scene is done in CGI.
 * One explanation for the difference in dialogue quality between the original trilogy movies (especially A New Hope and The Empire Strikes Back) and the new trilogy is that the dialogue was either ad-libbed or "improved" on by either Harrison Ford and the other actors more or less on the spot or the screenwriters who collaborated on the script.
 * The improvisation/script alteration was remarked on by Mark Hamill in an interview around the time of the film's release. Apparently Harrison Ford had covered his script with alterations so that he could say the lines his way, and this encouraged Mark to alter some of his own lines. The "prisoner transfer from Cellblock 1138" was Mark's ad-lib (instead of a random string of numbers), which Lucas didn't originally want to use since it was a blatant Shout-Out to his earlier film, but got put in the final cut.
 * There is at least one instance of a "throw it in" in the new trilogy. Hayden Christensen and Natalie Portman improvised the dinner table scene in which Anakin mentions "Aggressive Negotiations". Apparently, Lucas didn't like the dialogue he had written for the scene, so he just told them to improvise. Portman later said that "it got inappropriate very quickly."
 * Also, in the same movie, the name of the benefactor of the Clone Army was intended to be Jedi Master Sido-Dias (who was a thinly-veiled disguise for Darth Sidious, and he was not even existent within the Jedi), but the scriptwriter made a typo (due to the d and f keys being right next to each other). George Lucas ultimately thought it was better, so they not only kept the typo as the actual name, they also rewrote the scene to reveal that Sifo Dyas had in fact died several years prior.
 * Anthony Daniels' entire performance as C-3PO in the original movie. Lucas figured he would just overdub the dialogue to get the characterization he wanted, then changed his mind after several people, including Mel Blanc, told him to go with Daniels' voice and interpretation.
 * Die Hard
 * Alan Rickman plays Big Bad terrorist Hans Gruber. When he can't get information from a character, he shoots him in cold blood without a second thought. Later, he tells the rest of the terrified hostages, "I wanted this to be professional, efficient, adult, cooperative, not a lot to ask; sadly, your Mr. Takagi couldn't go along, so he won't be joining us for the rest of his life." This line was an ad-lib by Rickman.
 * He also ad-libbed the idea of eating some of the food from the party buffet while saying the line.
 * While not as spur-of-the-moment as many examples, the scene where Gruber pretends to be a hostage was written after the filmmakers discovered that Rickman could do an excellent American accent. The filmmakers had been looking for a way to have McClane and Gruber meet face-to-face before the movie's climax, and Rickman's accent provided a way to do that.
 * In one scene McClane tries to jump between air ducts in an elevator shaft. He misses his mark and just barely clings to one of the lower ducts - which was really an accident by the stuntman, but included in the final cut because it looked authentic.
 * In Monty Python's The Meaning of Life, Michael Palin ad-libbed the line "Hey! I didn't even eat the (salmon) mousse!" Of course, this does destroy the logic of the scene. As if it matters. And one of the extras during the end of the Mr. Creosote scene is actually vomiting.
 * It is even funnier because it broke the logic. That part was quite serious, until that line.
 * In Monty Python's Life of Brian, when Brian is telling everyone that they are all individuals, and they mindlessly repeat it, the one guy who goes "I'm not!" is an extra who just threw that out there on the spur of the moment. He got a pay raise to speaking actor.
 * Monty Python and the Holy Grail
 * The "He hasn't got shit all over him" line was improvised.
 * John Cleese has an improvised moment in the Burn the Witch scene; when asked why witches burn, the crowd is stumped. Cleese has the next line: "Because they're made of wood?" However, according to the DVD commentary with Eric Idle, he experimented with the timing between the question and the answer, even going so far as to start answering and then go back to thinking. Watch Eric Idle in this scene; towards the end of the pause he's biting down on his scythe to keep from laughing.
 * The line, "There are those who call me... Tim?" According to some versions of the story, the Enchanter did have a more appropriately mystical name, but Cleese forgot it while shooting.
 * In general however, the Python troupe rarely used ad-libbing.
 * The Right-Hand-Cat in the opening scene of The Godfather was not in the script. It was just some random stray cat that Marlon Brando befriended, and argued Coppola into letting him work it into the scene. And it works. In the same movie Lenny Montana, playing Don Corleone's henchman Luca Brasi, actually flubbed the line where he congratulates Don Corleone on his daughter's wedding. Coppola liked it, and inserted a scene earlier in the film, where Brasi is rehearsing his congratulation.

"Agent K: Does this come standard? Agent J: Actually, it came with a black dude, but he kept getting pulled over."
 * Legend has it that Lenny Montana (who worked for the Colombo crime family) was one of the thugs sent down to the set to see how the movie portrayed the Mafia, and whether changes needed to be made to the script; one of their demands, for example, was that the word "Mafia" not be used. The actor playing Brasi had had a stroke, they needed a replacement, and Lenny got the part. He was a big fan of Marlon Brando, and flubbed the lines because he was so nervous about meeting him.
 * Clemenza's now famous "Leave the gun, take the cannoli" line was a half-improvisation by Richard Castellano; the gun part was in the script, the cannoli part was not.
 * The famous classical music from 2001: A Space Odyssey was just supposed to be a placeholder used while they edited the movie. But Stanley Kubrick liked it so much that he kept it in as the movie's score.
 * In the Armageddon, Bruce Willis improvised the famous line: "The President of The United States just asked us to save the world... anyone want to say 'no?'" Michael Bay liked it so much he made sure they put it in the trailer.
 * A surprising amount of the jokes in Superbad were purely ad-libbed by the cast, usually until something was funny enough to cause the actors to break down laughing, and thusly added to the script.
 * In Spider-Man 3, Topher Grace ad-libbed the quip, "My Spider Sense is tingling, If You Know What I Mean." Apparently, he also added all sorts of jokes that didn't make it in, such as a needlessly creepy "I've just upgraded from a vanilla to a strawberry!" addressed at the red-headed Mary Jane.
 * In Atonement, director Joe Wright reveals in his commentary that the scene just before Robbie discovers the school girls massacre, at the point where he removes his helmet, the weather is cloudy. As he looks up the sky, the sunlight surprisingly shines and gets cloudy again the moment he put his head down.
 * In The Hours, when Meryl Streep goes to the sink and turns it on, the faucet explodes and shoots water up into the air; Meryl just went with it, and they kept the take.
 * Jaws
 * "You're gonna need a bigger boat." (both the book author and the screenwriter said they'd give an arm for writing such a line instead!)
 * Of the two shooting stars that appear during the shark's night attack on the boat, the first one was apparently real and kept in due to being a real one-in-a-million shot.
 * When the barrel whips over the front of the boat and knocks Brody's glasses off, it wasn't meant to get that close to Roy Scheider, and his reaction was at least partly natural.
 * The footage of the live shark thrashing around in the cables supporting the cage was captured when the animal accidentally got stuck there. This contributed to  as legend has it the dwarf actor they were using for purposes of scale refused to get back into the cage afterwards!
 * During the first take for Quint's Indianapolis speech, Robert Shaw was extremely drunk. They reshot the scene with him sober, but Shaw's performance in the first take fit so well with Quint's character, that the crew actually edited cuts from both takes into the scene.
 * Austin Powers in Goldmember examples:
 * During the fountain scene, at one point Austin's, ahem, "stream" starts giving out intermittent splashes like a sprinkler. According to the DVD commentary, this was actually a result of the water cannon malfunctioning, but the directors found it so funny they left it in.
 * Similarly, Scott Evil's little dance at the very end after gloating that he'll get Austin Powers in the same film was not in the original script. Seth Green, the actor for Scott Evil, was just fooling around with the set without realizing that the camera was actually rolling, but the directors found this to be hilarious so they left it in.
 * In Serenity Mal's "Faster! Faster would be better!" is such a Whedon line. It turns out it was ad-libbed, when Nathan Fillion was asked just to "say something Mal would say."
 * A Knight's Tale
 * The rather strange opening sequence was filmed on the second unit as a joke, and then the director decided to use it for real.
 * The scene where a crowd fails to cheer until one of the main characters does. This happened because the extras were all Czech, didn't understand English, and at first actually didn't realize they were supposed to cheer.
 * Also, in one shot where in response to how to beat William, Adhemar's page says that "With a lance, on a horse..." "...he's unbeatable." This shot was actually the result of a cameraman not realizing he was supposed to be doing a close-up until right as the action began.
 * Peter Sellers: according to IMDB, Inspector Clouseau's "rit of fealous jage" line in A Shot in the Dark was an actual slip of the tongue by Sellers. It was so Clouseau-esque, however, that Blake Edwards kept it in.
 * Doctor Strangelove
 * General Turgidson's tumble in the War Room was unscripted and accidental.
 * George C. Scott didn't intend to play Turgidson quite so whacky all of the time. In some of the scenes, after the "official" take, Kubrick would tell him to do it really, really over-the-top to amuse the other castmembers, then stuck that take into the final cut. Scott was initially upset about this until he actually saw the result.
 * A great deal of Peter Sellers' performance is said to have been improvised, including prominent examples such as President Muffley's "Just as sorry as you are" phone conversation with Premiere Kissov, and the title character's uncontrollable hand.
 * Malcolm McDowell claims Alex's use of the song "Singin' in the Rain" during the rape scene in A Clockwork Orange was an improvisation on his part which Kubrick approved. During rehearsal, the scene had not been working as scripted, so Kubrick told McDowell to try dancing. While trying this, McDowell spontaneously began singing the song. Kubrick realized it worked and immediately left the set to call New York and secure the rights to the song.
 * Speaking of Singin in The Rain, this also happened in-universe: Don Lockwood figured that the line he was supposed to say in his and Lena's first talkie when seducing her sounded too cheesy, so he decided to stick with repeated uses of the phrase "I love you" while kissing her arm. It... didn't quite work as well as they had hoped.
 * The crowd rushing the stage during the "Pinball Wizard" number in Tommy was not scripted, in one of the most spectacular "throw it ins" ever.
 * In the romantic classic An Affair to Remember, several of the dry-witted exchanges between Terry and Nickie were ad-libbed by Deborah Kerr and Cary Grant.
 * In The Outsiders, the scene where Dallas falls out of his chair while flirting with Cherry at the movies was an accident. You can see C. Thomas Howell briefly look at the camera.
 * The Dark Knight Saga
 * The Joker clapping sarcastically with everyone while in his cell when Gordon was promoted to Commissioner was an improvisation by Heath Ledger that Nolan immediately told the camera crew to keep filming.
 * In Citizen Kane, Joseph Cotten mispronounced the word "criticism" and quickly corrected himself, due to his exhaustion in acting in the ambitious film at the same time that he was starring in The Philadelphia Story on Broadway. Since his character was drunk in the scene Wells decided to use that take, and in fact you can see that he is initially surprised and then pleased by the mistake. There is a story that Welles intentionally had Cotten kept awake for a very long time, because extreme fatigue resembles drunkenness.
 * Probably the most bizarre example on this page: this gargantuan Narm from Shark Attack 3: Megalodon was a John Barrowman ad-lib, to try and make his co-star laugh. They "kept it in the freaking movie!". Why? Well, honestly, what did they have to lose?
 * In Max Mad 2: The Road Warrior, one stunt involved a motorcycle hitting an embankment, whereupon the stuntman was to flip over the handlebars and land on his back on padding in a standard stunt move. The stunt went wrong, the stuntman flipped head-over-heels two or three times, and wound up breaking both legs. The scene made it into the final film without reshooting, because it was that awesome.
 * The lineup scene in The Usual Suspects was scripted as a serious scene, but the actors didn't play it as such. Bryan Singer was initially pissed off about it, but ended up using some of the funniest takes in the final film. (so much that there's a take of everyone laughing) And in the scene where Redfoot the Fence flicks a cigarette into McManus' face, the reaction is entirely genuine: he was supposed to be aiming for the chest.
 * In the French movie Il y a des jours et des lunes, a priest who acts in amateur plays is at one point complimented on his acting skills. The actor playing the priest was supposed to answer with a joking "You're telling me you want to be my agent?" but flubbed the line into "You're telling me you want to be my apostle?" When he realized, he started laughing hysterically but tried to stay in character by apologizing and talking about Freudian slips and blasphemy before repeating the real line. The director decided to keep it because the slip was just too good.
 * In Star Trek II the Wrath of Khan, the design of the Starship Reliant was not supposed to look like it did in the movie: the visual effects team sent the design sketch to producer Harve Bennett for approval, and he signed off on the sketch upside-down. The vis-effects people realised that it actually looked better upside-down, and was more distinguished from the Enterprise with the nacelles angled down rather than up, so they built the model according to the "upside-down" view. That design of starship has been used in subsequent movies and shows.
 * In Star Trek III the Search For Spock, when Kirk learns the Klingons have  he seems to be so deep in shock he completely misses his chair and falls on the floor. Director Leonard Nimoy wasn't sure if Bill Shatner simply made it up or if he really missed the chair, but he felt that it was perfect for the scene and left it in.
 * For his part, Shatner confirms in his book Star Trek Memories that he did, in fact, just miss the chair and fall on his ass by accident.
 * In Star Trek 2009 McCoy's "All I've got left are my bones" line is an ad lib as is Scotty's "can I get a towel." You can see Spock's lips twitch after that one since he's trying not to laugh.
 * One morning in 1986, San Francisco native Layla Sarakalo discovered her car had been towed because the public parking space had been made available for a film crew truck. She figured the best way to get money to pay the towing fee was to work on the film that day as an extra. She managed to get hired that day to join the other extras. She was feeling a bit nervous, having never worked on a film before, so the other extras told her to "act naturally". When she got stopped by a "Russian" asking her how to get to the naval base in Alameda where the "nuclear wessels are", she naturally responded, "Ooh, I don't know if I know the answer to that. I think it's across the Bay. In Alameda." The director, Leonard Nimoy, loved that moment so much, he threw it in and it became one of the most frequently broadcast clips from Star Trek IV the Voyage Home.
 * Although the way Leonard Nimoy tells it in his autobiography I Am Spock, that shot was really just Walter Koenig and Nichelle Nichols accosting random passers-by and asking them about "nuclear wessels". Sarakalo was apparently just passing by, listened, and gave the instructions, and it's that shot which remains in the film -- they made a contract with her afterwards. One genuine ad-lib in there is the impassive police officer being asked questions: he was a genuine police officer there to provide security, and his reactions were just recorded on film and got in there.
 * In Ghostbusters, the appearance of Gozer was a last-minute decision by Ivan Reitman, made and announced to the cast right before the scene was filmed. Gozer's appearance in the script is that of Ivo Shandor, the occultist who began the summoning in the 1920s, but Reitman spontaneously came up with the idea of Gozer as an androgynous, otherworldly female, and her costume, equally improvised at the last second, is literally taped-up bubble wrap. The cast thought it was insane and would spoil the finale, and continue to express amazement today at how well it worked. Then again, Paul Reubens was intended to play the part of Ivo Shandor...
 * Bill Murray reportedly adlibbed at least some of his lines. The degree varies between different accounts from practically everything he said to just a couple of lines.
 * The commentary notes that practically every scene had an ad-lib, not just by Bill Murray either. Rick Moranis also ad-libbed much of his dialogue, especially in the party scene, though he worked with the screenwriters to get a vague outline of what was needed.
 * According to the DVD info for Black Knight, one of the female lead's faceplants was entirely unintentional, but kept in because it looked awesome.
 * While he was writing Boogie Nights, Paul Thomas Anderson accidentally mixed up two words while writing dialogue about Little Bill's cheating wife. He decided to leave it in, as Little Bill is angry when he says the line and would have mixed the words up.
 * The scene with Thorton Melon's secretary taking notes for him in Back to School was supposed to show his son Jason sitting next to her looking disgusted, but the actor simply couldn't stop laughing at Edie McClurg's performance. They decided to leave it in since it works just as well that he is supposed to be laughing in frustrated disbelief instead.
 * Norman Mailer directed a small number of films where he threw in unplanned bits:
 * In Wild 90, an improvisational movie, about twenty-five minutes of the soundtrack became muffled due to a technical error. Mailer decided to release the movie with the soundtrack muffled, rather than redub it, saying it "sounds like everybody is talking through a jockstrap."
 * During the filming of Maidstone, a movie about a director attempting to become President, Rip Torn attempted to hit Mailer over the head with a hammer. The two of them then fought viciously, all while the cameras rolled. The fight appeared in the movie.
 * While making Tough Guys Don't Dance, Ryan O'Neal gave a poor line reading which Mailer put into the movie, over the protests of various people, including O'Neal himself, because he felt the poor reading added something to the picture. See it for yourself here.
 * In Young Frankenstein, Marty Feldman started covertly switching his prosthetic "Igor hump" from shoulder to shoulder between scenes, until someone on the production crew finally noticed, and a bit was added where Frederick notices the change on-camera. Also, the "cat hit with a dart" sound-effect was Mel Brooks's on the spot improvisation.
 * Nearly all of Spaceballs was co-written by Mel Brooks, but the lines for the scene where Dark Helmet plays with his action figures? They were ad-libbed by Rick Moranis, who plays Helmet in the movie.
 * Another continuity reference happens at the end of El Dorado when John Wayne snaps at Robert Mitchum for alternating which arm he put his crutch under. Before becoming a big star, Wayne did part-time continuity work in college.
 * While filming The Princess Diaries, Anne Hathaway slipped and fell on her butt while filming a scene on some bleachers. Garry Marshall put that cut in the movie as her character was a Dojikko anyway. You'll notice Heather Matarazzo briefly slips out of character when this happens.
 * In the 2004 Dawn of the Dead film, an early scene filmed with a fixed camera on a car roof as it drives around the wrecked apocalyptic neighborhood almost caused an accident when the car drove in front of another vehicle, forcing the other driver to slam their brakes and barely avoid hitting it. Everyone agreed that it added a great touch to the apocalyptic feel of the sequence.
 * The scene from Mystery Men in which a burning trash can suddenly flares up behind the Spleen who jumps in surprise, then sheepishly says "Excuse me" to the other characters was a total improvisation on the part of Paul Reubens. Apparently, one of the workmen on the set didn't know the garbage can was a prop that would be later set on fire, and had thrown a disposable lighter into it.
 * The word "Shpadoinkle" from Cannibal! The Musical was originally a placeholder for the song "It's a Shpadoinkle Day," but when Trey Parker first played the song for friends they loved the word so much that it was kept.
 * There was also a scene where the town's drunken sheriff informs Packer that he's going to be hung at sunrise, then adds "You know what they say about sunrise?", awkwardly pauses for a moment, and just wanders off. According to the commentary, the actor (who actually was drunk) had just forgotten his line and walked off camera -- Trey Parker opted to leave it in because he decided that it was funnier than the actual punchline.
 * In Being John Malkovich, in the scene where John Malkovich is walking away from John Cusack, a passenger in a car driving by yells "Hey, Malkovich, think fast!" and throws a beer can at Malkovich's head and hits him. Apparently, it was an extra that had gotten drunk and just decided to do it at the spur of the moment. It was kept in, and the extra got a pay raise because he now had a line of dialogue in the film.
 * "It just be rainin' black people in New York!", said by Edwards as he drops onto a tour bus in Men in Black, ad-libbed by Will Smith.
 * Another Will Smith ad-lib made it into the sequel, when Agent J first shows Agent K the car's new "autopilot," a life-sized human model that pops out of the steering column;

"Rollo Lee: Oh, Wanda...! Willa Weston: Willa. Rollo Lee: --Willa...!"
 * Almost all of Tommy Lee Jones' lines in the first film were ad-libbed. Jones hated his character's lines, so he made up his own. Will Smith is genuinely confused half the time.
 * Harold Lloyd's silent 1928 comedy Speedy climaxes with a high-speed chase through the streets of NYC by a horse-drawn trolley. At one point during location shooting for the scene, the trolley crashed into a pillar holding up an elevated railroad platform. This was kept in the final film.
 * Randall's line in Clerks 2 of "What smells like shoe polish?" in the scene where Dante is painting Becky's toenails was meant to be "What smells like nail polish?" The "shoe polish" was a callback to the first Clerks.
 * Roy Batty's "tears In rain" speech from the ending of Blade Runner was actually a mostly-improvised performance by Rutger Hauer that was kept in the final product. According to Hauer and screenwriter David Webb Peoples, the script called for Batty to deliver a two-page speech that explained all of his past adventures in greater detail. After a long night shooting, and with the sun coming up on the final day of filming, Hauer (who had been trying to figure out how he could condense it down) stripped the speech down to its barest minimum and delivered it in one take.
 * When The Narrator exits from in Fight Club, he threatens the police officers with a "lead salad." The line was improvised by Edward Norton on the spot, as were the dubious expressions of the threatened officers.
 * According to the DVD commentary, during the first fight between the Narrator and Durden, where Durden is taunting him to "hit me, hit me in the face" and gets punched in the ear instead, Norton was supposed to take a swing at Brad Pitt's shoulder, but improvised and actually hit Pitt in the ear, resulting in the "Ah, God! Fuck! Why the ear, man?!" line.
 * The scene with Pitt and Norton hitting golf balls into the trainyard had nothing to do with the original script. The two of them were drunkenly aiming golf balls to hit the catering trucks; Fincher decided to film it and put it in.
 * From Tom Jones, according to the IMDB: "Hugh Griffith was reportedly drunk through much of the production; the scene in which his horse falls on him was not planned, and many believed he was saved by virtue of his inebriated condition. The film incorporated every frame of footage before rescuers entered the frame to save him."
 * In Almost Famous, Penny asks William if he wants to go with her to Morocco, and he answers, "Yes. Wait, ask me again." Penny asks him again, and he responds with an even more enthusiastic "Yes!" This was not in the script; Patrick Fugit, who played William, was simply asking Kate Hudson to do another take of that line, and Cameron Crowe left both takes in.
 * In Field of Dreams, Shoeless Joe Jackson accidentally hits a ball close enough to make Ray Kinsella jump aside with a "Whoa!" It also knocks over the nearby bag of baseballs.
 * "I have come here to chew bubblegum and kick ass, and I'm all out of bubblegum." from They Live!!
 * In Taxi Driver, the scene where Scout dances with Iris while playing soul music was based on an improvisation Harvey Keitel came up with while rehearsing. He asked Martin Scorsese to include it in the movie, because it added so much to the character. Scorsese was reluctant to do that because the rest of the movie is from Travis Bickle's point of view, but once he realized Travis could be outside the apartment watching from his taxicab, the scene stayed.
 * Also, "You Talkin' to Me??"
 * In the original 1962 The Manchurian Candidate, the scene where  has Major Marco slightly out of focus. Director John Frankenheimer later claimed to have heard theories this was done intentionally to show Marco from Shaw's hazy, brainwashed point of view. In fact, it was a technical glitch. They had tried to reshoot the scene with the camera in focus, but according to Frankenheimer, Frank Sinatra's performance was at its best in the first, out-of-focus take, and it deteriorated in take after take. In the end, they decided to use the take with the best performance, out of focus or no.
 * In the Transformers series Michael Bay is known for encouraging improvization among the actors, which led to Steven Spielberg talking to the cast saying he would be looking at the dailies and saying "That's not in the script." Apparently in the first film, the reason Mikaela was mostly looking away from Sam while he was driving her home is because Shia LaBeouf improvised this long line of dialogue where she wouldn't recognize him because he lost 100 pounds at fat camp and the friends he met there have died from diabetes. Megan Fox could not keep a straight face.
 * It's also done occasionally to get more realism, such as the dialogue on the AWACS, which was improvised by the crew based on what they'd say in a combat situation like that (only without the giant robot scorpion...).
 * The final scene from I Am a Fugitive from a Chain Gang had Paul Muni disappear into darkness as he said his final line "I steal," thanks to the lights being turned off a bit too early. Everyone agreed it was the perfect touch to end the film on.
 * At the very end of Martin Scorsese's film adaptation of The Last Temptation of Christ, in a scene depicting Jesus' Crucifixion, the film image suddenly dissolves and then goes stark white, as if there were a sudden light leak in the camera while they were filming. Turns out that that's exactly what DID happen - something had gone screwy with the camera while they were filming the scene, and no one noticed until they reviewed the footage later. But since it happened at precisely the point of Jesus' death in the film, Scorsese kept it in.
 * In Kill Bill, Daryl Hannah went off-script when she started screaming and flailing around in the trailer after the Bride vs. Elle battle. Apparently, Tarantino liked it.
 * During the filming of the chariot race in Ben-Hur, Charleton Heston's stunt double Joe Canutt almost flew out of the chariot when it jumped over a wrecked chariot, which was unintentional. The shot was left in with director William Wyler shooting a closeup of Heston climbing back into the cart. Reputedly, the crowd flooding the arena at the end of the chariot race was an unplanned move by enthusiastic extras.
 * Blazing Saddles: When Jim the gunfighter is telling Bart why he'll never be accepted in Rock Ridge, Gene Wilder's line goes, "These are simple folk, people of the land. The common clay of the new West. You know: morons." Cleavon Little starts breaking up, because Wilder had improvised the "morons" part.
 * In Walk the Line, Joaquin Phoenix as Johnny Cash rips a sink out of a wall while portraying Cash as being affected by drugs. The sink-rip was not in the script.
 * In The World Is Not Enough in the scene where Renard is told by Bond that  the actor Robert Carlyle's make up is actually slipping, but the director thought his performance so powerful that he kept the shot in. It's actually easy to mistake the slipping make up for tears, such is how it comes across!
 * Also, Pierce Brosnan ad-libbed the bit where he adjusts his tie during the boat chase at the beginning of the movie.
 * The bit in Scream where Billy goes to give Stu the phone, but it slips out of his hand, hitting Stu was an accident. Matthew Lillard screamed out "You hit me with the phone, dick!" The moment made director Wes Craven laugh so hard, he chose to keep it in.
 * Also, in the climax when Billy is attacked with an umbrella, his screams are real as the stuntman had hit a implanted wire in his chest. The wire was from heart surgery he got as a kid and touching it causes him immense pain.
 * Also from Scream 3 (and likewise in the climax), when Roman is searching for Sid and tries to find her with his phone only for her to beat him to the punch, distracting him. She pops up from behind a bar and stabs him in the shoulder with an ice pick. The scream from actor Scott Foley is real as she had missed the pads and actually struck flesh.
 * Deliberately cultivated in The Wind That Shakes the Barley, where the actors were given very little rehearsal time, and much of the dialogue consists of them interrupting each other or stumbling over words.
 * Christopher Walken's trick shot in Poolhall Junkies was accidentally filmed. As he was being taught how to perform it, he tried for the first time as practice and sunk the shot. They were filming, at his request, since he was afraid that he would be unable to sink the ball in any following take. If you watch you can see all the actors in the scene gasp and begin to laugh, even Walken looks surprised.
 * While filming White Heat, the crew ran into a problem. The scene takes place in the prison cafeteria, where Cody Jarrett has just been informed of his mother's death. As written it was falling flat until Jimmy Cagney seated the two biggest extras on either side of himself and told the director to follow him with the camera no matter what.
 * While filming Sweet Sweetback's Baadasssss Song, Melvin Van Peebles obtained a permit to set a car on fire for a scene he wanted to shoot that weekend. Unfortunately, he got the permit on a Friday, and the city hadn't filed it by the time shooting was scheduled. He did the scene anyway and when the fire department showed up, he filmed it and left it in the finished movie.
 * During the graveyard scene in Zoolander, right after Prewitt explains why male models are trained to be assassins, Ben Stiller completely forgot his line, and tried to wordlessly re-start the take by repeating his earlier line of "But why male models?", which prompted David Duchovny to run with it and hilariously reply, "...You serious? I just told you, like a minute ago."
 * From a scene between John Cleese and Jamie Lee Curtis at the end of 1997's Spiritual Successor to A Fish Called Wanda, Fierce Creatures (bonus points for Curtis' very visibly suppressed laughter immediately afterwards):

"Gerard: Newman, what are you doing? Newman: I'm thinking. Gerard: Well, think me up a cup of coffee and a chocolate doughnut with some of those little sprinkles on top -- while you're thinking."
 * In Pretty Woman, there's one scene where Richard Gere is showing Julia Roberts' character a very expensive necklace in its open box. The original scene as written simply required Roberts to tentatively touch the necklace and say it was beautiful. Instead, as Roberts touched the necklace, Gere clapped the box's lid down on Roberts' hand, scaring the hell out of her and making her laugh. Because of the way it looked on film, the take was left in -- watch where Roberts turns to when the joke is played on her -- she's looking offscreen at the crew, not anywhere near a camera.
 * In the film version of East of Eden, Cal was supposed to deck Adam after he rejects his present. When shooting, James Dean had the impulse to instead hug Raymond Massey. This became a moment of Enforced Method Acting for Massey who, unpracticed in improvisation, came across exactly as stiff and uncomfortable as Adam ought to be under the circumstances.
 * When Kevin Kline's character in A Prairie Home Companion opens the bottle of champagne, it was director Robert Altman he hit in the forehead with the flying cork. Kline's "Sorry!" was unscripted.
 * "Heeeeeeeeeeeeeeere's Johnny!" was reportedly not in the original script for The Shining, but was just Jack Nicholson adding in something to make Jack Torrence seem just that little bit crazier. And thus, a legend was born.
 * According to legend, Bela Lugosi was just beginning to learn English during filming of Dracula. He learned all his lines phonetically, and his odd mispronounciations have since become a very well-remembered part of the character. At least one reviewer has also noted they give the impression that Dracula isn't used to speaking at all.
 * In reality, he had been in the USA for eight years by this point, and spoke the heavily accented but serviceable English he would for the rest of his life. However, it is entirely possible that two years earlier, when he took the role on Broadway, he was directed syllable-for-syllable and kept the strange results.
 * Two flubbed lines by Robert Redford made it into the final cut of All the Presidents Men, thanks to Redford's ability to work the mistake into his performance. One was Redford on the phone with a person who spoke only Spanish, asking the others in the newsroom: "Does anybody here speak English?" when he was supposed to ask for someone who spoke Spanish. The other is at the end of a six minute take with Redford on the phone (again). He calls the person he's talking to by the wrong name but keeps going.
 * In the final shot of Barton Fink, the seagull diving into the water was unplanned.
 * Robert Englund improvised quite a few of Freddy Krueger's one-liners, but the best-known example happened in A Nightmare on Elm Street 3: Dream Warriors, in a scene where Freddy emerged from a television set and killed a girl by smashing her head into it. The scripted line was "This is it, your big break in TV!" which Englund said on the first take. When the director went for an alternate angled shot however, Englund changed the line to "Welcome to Prime Time, bitch!" The different camera angle made it easy to edit the two lines together, and it became probably Freddy's defining one-liner.
 * According to The Other Wiki, the line was originally "You're on TV now, girl!"
 * In The Fugitive, when Richard Kimble pleads with Deputy Marshall Gerard, "I didn't kill my wife!", Jones ad-libbed his blunt response of "I don't care!"--which promptly became the film's most memorable line.
 * Also, when Gerard and an extra are hanging around:

""Well, what do you think?" ["I don't think it will ever fly again.""
 * Harrison Ford injured his knee during filming, but postponed surgery until filming was complete, feeling (correctly) that the resulting limp would heighten the tension of the chase scenes and emphasize Kimble's vulnerability.
 * Similar to his actions while filming A New Hope, he also refused to memorize the script for the scene where he's interrogated by the police, allowing his responses to be completely realistic.
 * According to an interview with Uwe Boll, the infamous Dave Foley full frontal nude scene in Postal was a result of this: Foley was sitting down wearing only a robe and Boll had merely instructed him to stand up. Apparently, neither of them anticipated that the robe would suddenly open. Boll found this so funny that he had to leave it in.
 * In Time Bandits, Katherine Helmond was supposed to play the role of the Mrs. Ogre (on the ship) in heavy prosthetic makeup, like her on-screen husband. She really wasn't looking forward to this, and suggested to Terry Gilliam and Michael Palin that it might be funnier to make the character a normal-looking woman who was actually stronger than her monstrous husband. To her relief, they went for it.
 * In Avatar, during a scene where Neytiri is attempting to teach Jake how to speak Na'vi, she gets annoyed at his mispronouncing the word "nari" and smacks him in the side of the head. This was a real smack from the actress, and was not in the script. They kept it in anyway.
 * In WarGames, when David, Jennifer, and Falken were rushing to Norad on the Jeep, the Jeep wasn't supposed to crash, but it was kept since it added more dramatic tension.
 * In American Graffiti the opening where Terry (Charlie Martin Smith) crashes his moped into a garbage can was an accident but left in.
 * Soviet director Leonid Gaidai often used this. One of the most famous examples is a scene from his comedy The Diamond Arm where a smuggler who got out of the sea notices that his accomplice got stuck on a tiny island several hundred meters away from the shore. The first smuggler, played by the veteran actor Anatoly Papanov, looks out into the sea, growls "Idiot!" and angrily spits out. In reality, the "Idiot" remark was addressed at the cameraman because he had filmed the scene incorrectly and because of this Papanov had to go back into the cold water. Gaidai liked the genuine anger of that remark, so it was inserted into the final cut.
 * In the 2006 movie Deja Vu, there's a scene where the Timey Wimey Machine is ramping up to full power. As they're trying to get it to work, the lead machine wonk played by Adam Goldberg yells at his colleagues "I need more cowbell!" Reportedly this was an ad lib by Goldberg that suited the scene so well they left it in, and somewhat amusingly, several reviews singled it out as one of the most entertaining moments of the film.
 * In The Great Escape during the Fourth of July scene, Goff's line "No taxation without representation" was an ad-lib, causing Steve McQueen to do a double-take.
 * There is a scene in Harry Potter and the Chamber of Secrets where Draco Malfoy asks Harry, disguised as Goyle, why he was wearing glasses, as Harry had forgotten to take them off. When Harry replies that he had been reading, the script originally only had Malfoy stare him down skeptically. Tom Felton decided to mischievously add the line "I didn't know you could read," instead, and it was kept.
 * In the same film, actor Jamie Waylett didn't realize his character wasn't supposed to participate in the Slow Clap at the end, so he stood up and was pulled down again by Tom Felton. They kept it.
 * Jason Isaacs improvised two lines in the film: the first, as he was leaving Dumbledore's office and felt it was un-Lucius-like to let Dumbledore get the last word, he turned to Daniel and sneered "Let us hope that Mr. Potter will always be around to save the day." Daniel similarly ad-libbed Harry's reply: "Don't worry. I will be." The second ad-lib was Lucius' curse cut short by Dobby; the script didn't mention any specific spell so he just recalled from memory the "Avada Kedavra" curse, which led to some fans wondering how Lucius thought he could be using an unforgivable curse on Harry outside of Dumbledore's office.
 * When Hermione meets Harry in the first film, she says "Holy cricket, you're Harry Potter!" According to Emma Watson, she ad-libbed the "holy cricket" part and Chris Columbus thought it was hilarious.
 * In the fifth film, the Trio cracks up at the end of the scene when Harry tells Ron and Hermione about his kiss with Cho. This was an instance of Corpsing, which David Yates left in because he thought it hit the right tone.
 * Voldemort's memetacular hug of Draco was improvised by Ralph Fiennes. According to Tom Felton, they did over twenty takes of that scene and Ralph Fiennes only did the hug once. They used it, obviously.
 * In Tim Burton's Batman, when exploring Wayne Manor with Vicki, Knox (Robert Wuhl) ad-libbed the jokes about the decorative suits of armor Wayne has.
 * Many, many, many, scenes of Date Night are all improvising, and ad-libed.
 * In the original Rocky, loan shark Tony Gazzo is talking with Rocky about Rocky not breaking the thumbs of one of his clients, when he pulls out an inhaler in mid-sentence and uses it. This unscripted action happened because the man playing Gazzo actually had an asthma attack at that moment, and the director liked how it made the scene more authentic.
 * The shot in which Rocky runs through through the market and someone throws him an apple was actually a member of the public trying to hit Sylvester Stallone as he was filming the scene, unbeknown to Stallone who at the time thought it was part of the filming. The director however liked the shot and kept it in the scene.
 * In When Harry Met Sally..., Billy Crystal's "Pecan Pie" monologue is largely improvised. Meg Ryan's bafflement is genuine Enforced Method Acting, and you can actually see her glance off-camera for a moment. Rob Reiner made "run with it" motions, Ryan stayed in the moment and it stayed in.
 * Similarly, Matt Damon's story about his brothers in Saving Private Ryan was ad-libbed. Tom Hanks does almost exactly the same thing as Meg Ryan in the previous example -- watch his gaze flit off-camera for a second, then a slow nod. Is that "Captain Miller hears you, Private Ryan", or "Tom Hanks hears you, Steven Spielberg: run with it"?
 * On the subject of Tom Hanks and Meg Ryan: In You've Got Mail, there's a scene where the Tom Hanks character, holding balloons in one hand and a bagged goldfish in another, accidentally closes the door on the balloon strings. In an ad-lib, Hanks re-opened the door to free the balloons and joked to Ryan, "Good thing it wasn't the fish!"; it made the cut.
 * R. Lee Ermey's Gunnery Sergeant Hartman, in Full Metal Jacket, pulled the "reach-around" line, when insulting a recruit from Texas, out of his head. Kubrick stopped the filming to ask Ermey what that meant. After it was explained, Kubrick simply said to go with it. Other Hartmann lines were also ad-libbed, with Ermey being one of the relative few that Kubrick, notorious in some circles as a control freak, allowed to go off-script.
 * It probably helps that Ermey really was a Drill Instructor during Vietnam.
 * Ermey's entire character in the movie was thrown in, sort of. Ermey sat down with Kubrick as a "technical adviser", and for hours straight he literally just had a stream-of-consciousness moment with every horrible thing he could think of when yelling at new recruits, for later incorporation into the script. Kubrick was so impressed with it all, he later decided to just cut out the middleman and cast him as Hartman. In a way, any time he ad-libs in the movie it's the same as an "executive rewrite."
 * In My Big Fat Greek Wedding, Toula's aunt, who goes up to Ian (Toula's fiancé) to ask him something, randomly says "Let me touch your hair", and begins to massage it. This was an ad lib on Andrea Martin's part, as she'd forgotten her line.
 * In the classic Bringing Up Baby, Cary Grant's character Dr. David Huxley has lost his clothes and is forced to find whatever he can around Susan's aunt's house to wear home. Naturally, all that is available are a ridiculous pair of hunting boots and a woman's frilly nightgown. When Susan's aunt sees him, she angrily asks him why he's wearing those clothes; impatiently, Grant jumps in the air and shouts "Because I just went GAY all of the sudden!" Whether this was a reference to homosexuality or not is unclear, but it wasn't scripted in any case.
 * It's 90% likely an intentional reference (also the first use of the term 'gay' to mean homosexual in a Hollywood movie): Note Huxley's next line is that he is "in the middle of 42nd street waiting for a bus"; at the time 42nd & Broadway was New York's seediest area, with lots of cruising homosexuals.
 * In Get Smart, the scene after the parachute jump in which 99 grills Max about what he would do if someone pointed a gun at him was taken almost verbatim from dialogue Anne Hathaway ad-libbed for her screen test. The director like it so much, he added it to the film. (From the making-of featurette on the DVD.)
 * According to multiple interviews by Gregory Peck, the famous scene in Roman Holiday in which he pretended to have his hand bitten off by the Mouth of Truth was ad-libbed by him, with only the director being aware of it in advance. Audrey Hepburn's scream and her relief laughter were genuine reactions. According to Peck, he borrowed the joke from Red Skelton.
 * In Return of The Pink Panther, Catherine Schell can be seen breaking into laughter at some of the antics of Peter Sellers. The two scenes in question are when Insp. Clouseau impersonates a telephone repairman, and later when Clouseau meets her in a restaurant and pretends to be a lounge lizard; in this latter example the scene ends with Schell choking on her drink. It's been said Schell's laughter (and the choking) were outtake-worthy moments that the director decided to keep; Schell has claimed they were scripted.
 * In Kramer vs. Kramer, the last scene of the movie where Joanna (Meryl Streep) asks Ted Kramer (Dustin Hoffman) "How do I look" and he replies "You look terrific" took place before the filming was supposed to begin, apparently Robert Benton liked it more than the original scene, and left it in.
 * Billy Wilder and I. A. L. Diamond struggled for days with the final dialogue between Jerry and Osgood in Some Like It Hot, trying to think of an appropriate answer from Osgood when Jerry reveals he's a man. Unable to think of anything funny, they gave up and had Osgood say "Nobody's perfect." This has gone down in film history as one of the funniest punchlines and film endings ever. Billy Wilder even used the sentence as the title for his own autobiography. It's also on his gravestone.
 * In Inception the infamous "Mustn't be afraid to dream a little bigger, darling" line was said as a joke, but Nolan liked it and decided to throw it in.
 * In Ghost Rider, the first scene with Nicolas Cage as Johnny Blaze, a professional daredevil, has him failing a jump. During the fall, the front wheel of his motorcycle smashes into his helmet, breaking the visor of the helmet. This was not intentional, and the stuntman really did take a tire to the face. However, when the stuntman saw the footage of the crash, he thought it looked good, so they decided to leave it intact.
 * Many scenes in The Lost Skeleton of Cadavra, including a malfunctioning spaceship door and Animala saying "click" aloud as she flips a switch, were deliberately left in on account of the Rule of Funny.
 * In Road To Morocco, Bob Hope and Bing Crosby are stranded in the desert when they find a convenient camel. In mid-line, Bob gets spit in the eye by the camel, and Bing laughs "Ho ho, good boy!" They pretty much had to keep that in.
 * In the Blaxploitation parody Black Dynamite, there is a scene where some men in black suits are shooting at Black Dynamite and a man in a large jelly doughnut costume from the car. As they pull up in the car and begin shooting, the car begins rolling away due to the fact that the actor forgot to enable the parking brake. He got it eventually, but the shot was kept in due to the fact that it fit in with the rest of the movie's intentional "Throw it In"s.
 * Supposedly, in Down Periscope, the scene where Lt. Emily Lake (played by Lauren Holly, as a Naval experiment for having women on submarines) confronts her commander (played by Kelsey Grammer) regarding a sub maneuver that he'd pulled to help her regain her confidence. When leaving, the part where the actress slammed her elbow into the doorjamb, and gave a short hysterical laugh before darting out of the set was not in the script, but kept anyway for extra laughs.
 * Also supposedly, many of Nitro's lines were ad-libbed. The character, portrayed by Toby Huss, was only supposed to have a couple lines in the beginning of the film.
 * In Attack of the Killer Tomatoes, the helicopter that they'd rented for the day crashed, and they caught it on film...so they added a line a about a tomato leaping at it.
 * The actors just got out of the wreck, dusted themselves, and went right on with the scene.


 * In Finding Neverland, at the Clap Your Hands If You Believe part, the uptight Emma starts clapping fervently. The children were shocked in response, since the actress wasn't supposed to. It adds a lot to the scene.
 * In The New Guy, Real Life twins Jerry and Charlie O'Connell improvised climbing on a swing-set to do upside-down crunches, hence the Romantic False Lead's very confused look.
 * In Kick-Ass, the entire bazooka subplot was improvised in the course of filming.
 * Similar to The Birdcage above, in John Carpenter's Vampires, John Carpenter got along with James Woods, a notoriously hard to work with actor, by allowing him to to ad-lib as long as he did at least one take strictly by the script. According to the DVD commentary, a lot of the ad-libs made it to the final cut.
 * During a car chase in RoboCop, a hubcap comes loose and rolls almost directly at the camera. Paul Verhoeven left it in since it looked very cool.
 * In The Sound of Music, Julie Andrews tripping at the end of "I Have Confidence" wasn't scripted, but was so perfectly in line with her character that it was left in.
 * In American Pie Jim asks Michelle, aka Alyson Hannigan, to the prom. After she talks about using her flute to make her sing and wanting to have sex with Jim, she climbs on top of him and says, "What's my name? Say my name bitch!" This was yet another "tried it differently on the last take" that they threw in. (You can see all the takes on the DVD special features.)
 * The scene in Wild Hogs where John Travolta attempts to pull off a poor Clint Eastwood impression in the biker bar was completely improvised on the spot. "What the hell is wrong with you?" wasn't directed towards his character.
 * In The Warriors, the famous line "Warriors, come out to playyyyyay" was stated to have been improvised by David Patrick Kelly, inspired by a man who used to make fun of him in New York. The use of the beer bottles was also an improvisation. (The original intention had been to use dead pigeons, but it didn't happen.) Good thing too, since that's the one scene everyone remembers.
 * In the classic Russian holiday film The Irony of Fate, or Enjoy Your Bath!, the Love Interest's fiancé Ippolit is thrown into a shower fully clothed in order to sober him up. After the water is turned on, Ippolit says "Oh, hot water, very nice!" Apparently, it took a while for Russian showers to warm up, so the actor was surprised when he got doused in hot water instead of cold. The director loved the unintentional ad-lib and kept it.
 * During the chase sequence in the original Gone in Sixty Seconds, the Mach 1 Mustang driven by the protagonist was accidentally clipped from the rear by another car, causing it to spin out of control and collide with a lamp post. The collision was kept in the film for dramatic effect.
 * In Scott Pilgrim vs. the World, Wallace bursts into the apartment drunk and throws his keys at Scott's head. The actor did this as a joke, but Edgar Wright loved it so much that he kept it in.
 * In Napoleon Dynamite, there is a scene where Napoleon attempts to hop a fence but ends up falling over onto the other side. Jon Heder actually fell while taking this shot, and the makers decided to keep it
 * In The Wind and The Lion, during one of Teddy Roosevelt's monologues, a horse lies down and rolls. In the commentary, the director notes that most filmmakers would have reshot the scene, but he kept it in for verisimilitude.
 * A lot of dialogue in Dog Day Afternoon including Al Pacino yelling "Attica! Attica!" and John Cazale's response when Pacino asks him what country he wants to go after the robbery: "Montana."
 * In the opening number of Gold Diggers of 1933, Ginger Rogers sings "We're in the Money. In between takes, the director heard Ginger joking around speaking fluent Pig Latin. He then decided to put in a part where the camera closes in tight on Ginger as she sings a verse of the song in Pig Latin.
 * In the final battle of Terror Of Mechagodzilla, there's a moment when (due to a nearby explosion) Godzilla's back-spikes catch fire. You can bet they left that shot in.
 * In the American Godzilla, there's an establishing shot of Manhattan from the south, in which an ominous bolt of lightning strikes one of the Twin Towers. It's totally real.
 * Word of God on Brad Silberling's director commentary for A Series of Unfortunate Events states that Jim Carrey ad-libbed quite a few of his lines during practice runs. His practice lines damn near perfectly added to the scene's mood almost every time and were memorable even when they didn't, so Brad shrugged it off and said, "Eh, what the heck." Thus, almost all of his best lines in the movie were actually cooked up during practice runs. Overlaps with Harpo Does Something Funny, because anyone willing to cast Jim Carrey knows he can make a scene absolutely perfect if you don't try to order him around too much.
 * In Orson Welles's Touch of Evil, a scene featured a shot of Welles smoking. A piece of paper accidentally blew by in front of Welles. It was kept in at his request.
 * In Terminator 2: Judgment Day, Sarah Connor, after breaking out of her cell, ambushes an orderly by whacking him out and inflicting cuts in the process. This was not acted: Linda Hamilton actually inflicted the orderly's actor with the injury as revenge, because he went too easy on her when she was being restrained in an earlier scene (causing James Cameron to re-shoot that scene several times, and she had to fall to her knees on a hard tile floor each time). It was kept in the final cut.
 * True Lies: Word of God says that Curtis's slip and fall during the stripper dance wasn't scripted, and you can even see Arnold jumping out of the chair to see if she's alright. She instead jumped right back up and continued the dance, with Arnold sitting back down quickly. Luckily, all of this is perfectly in character (Harry would obviously be concerned about his wife, and then hastily attempt to maintain The Masquerade when the show goes on) and it ends up as one of the funniest scenes of the movie.
 * In The French Connection, the script called for a number of near-misses during the chase scene, but in practice, many of them were mis-timed, resulting in accidental collisions that were left in for realism.
 * In Teenage Mutant Ninja Turtles II, Michelangelo accidentally drops a piece of his pizza into the canister of anti-mutagen. You can tell this was unscripted by the way Mikey immediately looks up at the camera with an Oh Crap expression on his face.
 * In the 2002 caper movie Stark Raving Mad, when Ben is knocking out with a convenient bottle, they had several takes where the Soft Glass bottle didn't actually break -- but the actors were so dedicated to selling every take that they just had Ben shatter the bottle and knock the guy out with the second blow.
 * Ferris Buellers Day Off has actress Edie McClurg's famous line, "They think he's a 'righteous dude.'" This was ad-libbed.
 * Mighty Morphin Power Rangers has a bit at the end of Ivan Ooze's rant about the horrible things inflicted on humanity he missed out on: "The Black Plague! The Spanish Inquisition! The Brady Bunch Reunion!" That last one was ad-libbed by Paul Freeman.
 * Literally the only explanation for the bulk of Pocket Ninjas; many of the things going on, especially during supposed fight scenes, only make sense if you assume the actors were clowning around without realizing the cameras were rolling and the director (who may or may not have been drunk and high at the same time) decided that that was exactly what he wanted in his movie.
 * In a scene from Clueless, Cher is giving a speech about granting asylum to Haitians. Cher pronounces it "Hate-ians" instead of "Hay-shens." This wasn't scripted - Alicia Silverstone didn't actually know how to pronounce Haitians. The director liked it so much that she told the crew not to correct Silverstone.
 * In Captain America: The First Avenger, when Steve finishes getting transformed by the super serum, Agent Carter briefly reaches out to touch his abs, before pulling back at the last second. This was not scripted.
 * In the 1990s remake of The Nutty Professor, Sherman Klump/Buddy Love and the comic's dialogue during the comic act was ad-libbed by Eddie Murphy and Dave Chappelle, respectively.
 * Most of Elliot T. Jindrake's character in Max Keeble's Big Move was ad-libbed by Larry Miller, wanting to portray him like the dean from Animal House.
 * Max Keeble's giggling when meeting Jenna at the middle school was a flub on Alex Linz's part, but the creators liked it and put it in.
 * While The Three Stooges were filming the train scenes for "Hold That Lion!", Curly Howard just happened to pay a visit to the set. Jules White saw an opportunity and improvised a scene with Moe, Larry, and Shemp harassing Curly as a snoring passenger.
 * The "Put on a phat beat for me to beat my buddy's ass to." line in the Iron Man party scene. It's pretty obvious (as Robert Downey Jr. immediately cracks up on camera), but pretty hilarious as well. (And it's in character as Tony Stark is supposed to be drunk.)
 * In the roulette scene of Lola Rennt, an initial take was filmed of the wheel spinning and the ball being dropped, with the intention of later editing it together with a staged shot of the ball landing on twenty to complete the scene and win her the money she needed. The ball landed on twenty in the first take.
 * One day, during the filming of Eternal Sunshine of the Spotless Mind, the circus came to town. So they filmed a scene at the parade.
 * The "Most Annoying Sound" scene in Dumb and Dumber was unscripted (you can tell because it's clear Jeff Daniels is about to crack up).
 * Unknown Island When a horde of Ceratosaurs descend upon the heroes, who are throwing grenades at them, a little mistake made it in for the better. Because they were filming in mid-day in the desert with heavy rubber suits on. An explosion went off near one actor, who then promptly fainted from the heat. They kept it in the scene.
 * Return of the Living Dead has one scene toward the end where . This is because James Karen didn't want to shoot his final scene in the cold rain and instead suggested that Frank because he's a nice guy and didn't want to hurt anybody.
 * At the end of Sea Of Love Al Pacino bumps hard into an approaching passerby while walk&talking. He gets hit so hard, that he's actually knocked back a few steps, yet doesn't even so much as blink, and fluently continues his speech. That wasn't scripted, in fact the guy wasn't even an extra. In the dvd-commentary, the director explained that they couldn't close off the whole location, it being a public street in New York, and that the pedestrian was real. It's realistic, because he's  so it's understandable that his character completely ignores it. Plus it's New York, people who live there probably don't even notice anymore.