Pushed in Front of the Audience

A situation in which one or more characters are unexpectedly asked (or compelled) to take an acting part in a Show Within a Show. Maybe they just happen to be present backstage when one of the real actors suddenly collapses, or maybe they get mistaken for the real actor for some reason and have to play along, maybe they literally drop on stage by accident while doing something else, or maybe getting on stage is the only way for them to get close to a MacGuffin. Whatever the cause, they'll find themselves out in front of everyone trying to pretend that they know what they're doing. They might have a brief opportunity to skim the script, or they might just have to make it up as they go along, but expect it to be awkward either way.

Most of the time, the show in which they're forced to act is a play or other live theatrical production, but it could also happen with other media.

It can sometimes happen in video games, which requires that the players themselves do a plausible job of remembering their lines.

A frequent cause of Bad Bad Acting. See also All Part of the Show, which is when characters aren't trying to follow their actual role, but the audience thinks they are anyway. When a character stands up to perform (as opposed to being pushed) because it's their best/only option, they are Holding the Floor. Frequently employed as part of the Concert Climax. May result in the Concert Kiss.

Anime and Manga

 * Kagome in Inuyasha; simultaneously Played for Drama and Played for Laughs in that there is a real youkai hiding in the building and Inuyasha is having trouble determining acting from reality. The audience, of course, presumes that it's All Part of the Show.
 * The third One Piece TV-Special focuses on a theather actor putting on one last show. When some of the cast suddenly quit right before the opening act, naturally the Straw hats ended up on stage causing all kinds of chaos.
 * In one episode of Samurai Champloo, Fuu throws the dice for an important gambling match when the person who was supposed to do it is kidnapped so that one of the participants can supply a substitute who will cheat for them.
 * In Hayate the Combat Butler, Hinagiku gets thrown on stage to sing "Cruel Angel's Thesis" for her own birthday party in chapter 95 or episode 12 of season 2. Wataru and Hayate both get their own turns during Sakuya's birthday in chapter 141 or episode 23 of season 2. They are forced to perform their own comedy shows.

Comic Books

 * Obelix in Asterix and the Cauldron.

Film

 * The "getting on stage is the only way for them to get close to a MacGuffin" version occurs in The Adventures of Sherlock Holmes' Smarter Brother. Sigerson Holmes and his Sidekick Sergeant Sacker go onstage as actors during an opera to recover the Redcliff Document. On YouTube here.
 * In the early Alfred Hitchcock film The 39 Steps, the main character gets mistakenly called to speak at a political rally and manages, without knowing what candidate or party he is supporting, to get a standing ovation. (This is inspired by a sequence in the original novel, but that plays out differently and doesn't quite fit the trope.)
 * In Jingle All the Way, Howard, running from a cop, is mistaken for a replacement for a Turbo Man actor, and is essentially shoved into the suit. As Turbo Man, his job is to give out a free action figure during a parade.
 * Moulin Rouge, the hero and The Dragon end up on stage during the climax of Spectacular Spectacular. In this case, the hero actually wrote the script, and nobody on stage wants the audience to realize there is a major catastrophe in progress with the show, so they collectively adlib around the fight going on on stage as the bad guy attempts to recover a gun he dropped (which the actors keep kicking away from him).
 * In Chitty Chitty Bang Bang, Caractacus Potts, running from an angry customer at the fair, ends up hiding in the middle of a men's song and dance act. At first he's obviously behind and trying to copy everyone else, but by the end, he has blended in seamlessly and ends up getting a cut of the money just like all the other guys who were actually supposed to be performing.
 * In Lets Get Laid, a shy recently demobbed (discharged) soldier meets an actress, and looks exactly like her leading man. At the end of the film, the actor gets arrested (wrongly) and the ex-soldier gets pushed on-stage on opening night. The resulting chaos is loved by the audience, and he's recruited to replace the original leading man.

Literature

 * Discworld
 * In Wyrd Sisters, the three witches (who have been keeping an eye on things) are rushed onto the stage by one of the crew members to play the part of the three witches. In the same book, walks onto the stage, expecting the Weirdness Censor will stop anyone noticing him. When he realises that the audience were expecting to see Death walk onto the stage, he forgets his line and needs prompting.
 * In Maskerade, after the witches realise that the villain they're pursuing is hiding in plain sight on stage.
 * Happens to the protagonist of The 13½ Lives of Captain Bluebear.

Live-Action TV

 * Happened on Monk once, when Monk is persuaded to take over the part of an actor whose death he is investigating. He did reasonably well during rehearsal, but missed his cue during the real thing, was pushed onstage, and ended up making a fool of himself. He redeems himself, however, by solving and explaining the murder onstage.
 * Technically show-within-a-show-within-a-show: once in the Square One TV SwaS "Mathnet" they solved the crime and approached the perp (who was a Broadway actress) right after the curtain came down - then the curtain went up again for the Curtain Call. So they do The Summation in song, to a standing ovation.
 * This is pretty much the point of Thank God You're Here, except that the performers know they're about to be put in front of the audience. What they don't know, however, is what the 'play' they're going to be in is about.
 * One Amazing Stories episode featured a mix-up between a movie actor dressed up as a mummy, and the actual walking mummy whose legend is being filmed. The real mummy winds up roped into playing the part (which isn't difficult, as there are no lines) while the actor nearly gets burned alive as a monster by superstitious locals.
 * In the Quantum Leap episode "Catch A Falling Star", Sam leaps into an actor about two minutes before the show starts, and is about to be pushed out on stage with no idea what's going on. Fortunately he's the understudy, and the real lead shows up at the last second.
 * An entertaining Star Trek: Voyager episode forced the Doctor to fake being in command of the starship and bluff the belligerent aliens into going away at the climax.
 * Psych did this on the season-2 episode "Lights, Camera, Homicidio." Shawn was investigating a murder on the set of a Telenovela, and he managed to become the show's breakout star, at least until he solved the crime and quit.
 * Done to Willow in an early "What Do They Fear?" Episode of Buffy the Vampire Slayer; she hallucinates being pushed on stage in the middle of an opera and she can't sing.

Newspaper Comics

 * In a Peanuts arc, Lucy volunteers Linus to sing "Jingle Bells" for the PTA assembly.

Theatre

 * In The Producers, the director of the Show Within a Show has to take a part when the person meant to be playing Hitler breaks his leg.
 * The Actors Nightmare, a short comedy play, is a somewhat surreal instance - not only does the protagonist have to act despite (as far as he can remember) not being an actor at all, he gets contradictory answers about what play is being performed, and it seems to keep changing as he improvises. He ends up in a distressingly realistic execution scene that might (or might not) actually cause his death at the end of the play.
 * The gangsters in Kiss Me Kate become part of the Taming of the Shrew production.

Video Games

 * Baldur's Gate II has an example, when the hero can end up owning a theatre. The lead actor comes down sick on opening night, and the player needs to remember his lines in order for the play to be a success.
 * Jade Empire gives the player the option of acting in a play which is being monitored by the authorities for possibly containing subversive messages. By their choice of lines, the player determines whether or not they find any (or they can just screw up, of course).
 * Neverwinter Nights 2 Mask of the Betrayer has a dream sequence in which you get shoved onto a stage to play the role of Akachi the Betrayer, going along with the whole 'masks and illusions' theme present throughout the campaign
 * The Morrowind expansion pack Tribunal gives you the option of replacing a sick actor in a politically charged play, and if you try it, someone tries to assassinate you part way through. Turns out that the assassin's presence was known, and you were deliberately put in that position in order to protect the actor.
 * There are some Final Fantasy examples:
 * Cloud and his date (usually Aeris, but not always) take part in a play at the Gold Saucer in Final Fantasy VII.
 * Then there's Celes performing at the opera in Final Fantasy VI.
 * Not to mention the bit where Locke and Ultros fall onto stage and knock out two vital main characters, and adlib themselves into the plot so they can duel for "Maria" (Celes). The Impresario is livid.
 * And in Final Fantasy IX, when Dagger crashes into the play being performed at the beginning of the game, she ad-libs a very nice exit scene. Justified since the play is the in-universe equivalent of Romeo and Juliet, and Dagger is a classically educated Princess who happens to love that particular play.
 * In Tales of Vesperia, the Acting Guild asks you to take parts in a play. If you accept, you are rewarded at the end with titles that change your party into their costumes from the play.
 * In Indiana Jones and the Fate of Atlantis, a knife thrower needs a volunteer assistant and Jones literally pushes Sophia in front of him.

Western Animation

 * Happens to Elmer Fudd in "The Rabbit of Seville." In "Stage Door Cartoon," Elmer Fudd and Bugs Bunny do this to each other.
 * They both end up on stage in "Slick Hare" - true to character, Fudd flees in panic, while Bugs happily goes into a dance.
 * In the Kim Possible episode "Hidden Talents", Ron goes onstage at a talent show to stall for time while Kim escapes from a Death Trap and returns. (Also, Kim was in the show to begin with because Ron signed her up without asking her.)

Real Life

 * This happened to Guy Goma. He was at The BBC Television Centre for an interview for a job in their IT department on May 8, 2006. Somebody thought he was Guy Kewney, who was there for a live interview regarding the Apple Computers v. Apple Corp lawsuit and dropped him in front of the camera. Now aware that he was in a live situation, he decided not to cause a scene and did his best to answer Karen Bowman's questions.
 * Which presumably inspired similar events in The IT Crowd when Moss, appearing on Dragon's Den under a pseudonym, is mistaken for an MP and taken into an interview about the Iraq war.