Suddenly Always Knew That

"Yes, the enlightened Captain Picard -- who loves playing flutes, drinking tea and reading Shakespeare -- also loves redneck off-roading."

—Mr. Plinkett

How did a character suddenly acquire a needed skill?

By an unrelated previous life experience he never knew would prove so useful, sometimes a Retcon. In other words, the character always had the skill, but You Didn't Ask.

A character who is an actor is a sharpshooter because he had to learn it for a role. A writer is an expert on medieval history because she had to research it for a book.

Common with older, gray-haired characters. You never think that your mother or grandmother could've had a life before you were born.

Combine this with Character Development to get Taught By Experience and Took a Level in Badass. See also I Know Mortal Kombat, Taught by Television and Instant Expert. Compare New Powers as the Plot Demands, where not even the character knew that he had the new skill all along.

See also Deus Ex Machina, a similar and often related concept. The polar opposite of this trope is Informed Ability. I Minored in Tropology is a Sub-Trope.

This trope used to be called "I Know Kung Fu" (a reference to a scene from The Matrix which actually exemplifies Instant Expert); that name now redirects to I Know Karate.

Examples of Suddenly Always Knew That include:

Anime and Manga

In Death Note, L and Light both somehow have pretty good hand-to-hand combat skills. L's skill is somewhat Retconned in Another Note, a novel that takes place before the story. Light we just assume has every skill necessary at the moment. He tends to have that. Oh! Watari's also a sharpshooter.
~~Wilford Brimley~~ Dr. Reichwein of Monster used to work with the border police, but the two punks trying to beat him up in an alley didn't know that.
- Averted with Nina, who was shown practicing Aikido before it came in handy.

Film

In Monsters vs. Aliens, when needing to break into the Big Bad's computer that uses a Dance Dance Revolution key pad, Dr. Cockroach reveals that his PHD is in "Dance!" and breaks the code.
In the film Three Kings, one National Guardsman suddenly reveals himself as an airport baggage handler in civilian life when a need for detailed knowledge of suitcases arises.
In The Poseidon Adventure, Shelley Winters' character, while seemingly the least fit (and least useful) of the survivors, was able to swim a long distance underwater while carrying the end of a rope, because she was on the swim team in school. Sadly, she dies of a heart attack immediately afterwards.
Small Town Indiana resident Tom Stall is much more skilled at hurting people than any coffee shop owner has any business being... Morbidly, it turns out there is A History of Violence behind his skills.
In the film Three Amigos, Ned Nederlander reveals that he actually is a real quickdraw artist when challenged by a German former fan. The German assumed that Ned used trick photography, but Ned insists that it's all him. Subverted by Ned later when he reveals that he learned to fly the German's exact make and model of bi-plane for a film, but his stuntman actually did the flying. He still manages to fake it long enough to escape.
The Other Guys uses this trope frequently.
2012 loves to use this trope. For instance, Gordon Silverman is a plastic surgeon who is also an amateur pilot. He is used to help fly a plane when the group needed a pilot. Of course, a day's worth of flight lessons in a two-seater Cessna doesn't qualify you to fly the diversity of aircraft he pilots in the film. Hell, it barely qualifies you to fly a two-seater Cessna.

Literature

Sherlock Holmes suddenly reveals himself to be a master of "baritsu", which enables the ultimate Author's Saving Throw.
In Jack Vance's Lyonesse trilogy, Aillas never comes across as someone particularly interested or skilled in swordsmanship. In The Green Pearl, however, he encounters an infamous bandit leader from a very martial culture who considers himself virtually peerless with a blade in hand. Out of nowhere, Aillas hands him a Curb Stomp Battle without breaking a sweat. A few chapters later, a bystander comments that he's a "demon with a sword."
Mk Venner, from Dan Abnett's Gaunt's Ghosts novel, really does know kung fu. Before joining the Imperial Guard he trained with a group of secretive 'woodland warriors' and also speaks fluent Old Gothic. Of course, you don't learn this until it saves the whole squad in His Last Command.
In Atlas Shrugged Dagny flies an airplane when she goes searching for John Galt. It is never mentioned why or how she knows how to fly.
The Longing of Shiina Ryo: When Shin-tsu has to defend himself from Kouma.

Live Action Television

In Buffy the Vampire Slayer, Xander (once mentally turned into a soldier by a spell) knows the layout and lingo for the local military outpost. When these turned out to be useful, the writers Hand Waved them away... and Xander didn't even tell us about that until months after his implanted memories were gone.
- In the seventh season, the writers seemed so desperate to justify Dawn's very existence that she suddenly could translate old magical books from sumerian. It was slightly less justified than the same character demonstrating sword proficiency in the last episode of the sixth season. At the time, she at least said she learnt that by watching her sister fight demons.
  - Though it's implied that she spent a very long time translating from Sumerian, meaning that she was probably slowly translating with the aid of dictionaries and other resources.
  - Also, she had ample opportunity to learn; Giles is a qualified instructor in ancient Sumerian (and many other obscure and ancient languages), and Dawn was right there with everybody else helping during the Scooby Gang research parties. All she'd need is a better memory than average -- which she has -- and she'd pick quite a lot up.
In Castle there there are several instances. At least Once Per Episode Castle brings up some random piece of knowledge that his research or various hobbies have taught him. He can also speak Mandarin, which he attributed to a TV show he used to love.
- Beckett can also slip into a Russian accent which she attributed to a semester she spent abroad.
In Charmed in "That Seventies Episode", Phoebe suddenly reveals the ability to pick locks. Lampshaded by Prue, who rhetorically asks, "Why am I not surprised that you know how to do this?"
Catherine, typically the classiest and snootiest character on News Radio, nonetheless knows a little three card monte from a summer spent hustling with an uncle.
- Dave Nelson also has a surprising amount of skill in both knife throwing and tap dancing.
  - Catherine Duke is a poor man's Dave Nelson. Dave Nelson also just so happens to be a ventriloquist, speaks Spanish, is an expert at the arcade game Stargate [Defender], can sing a cappella, and can cross dress surprisingly well.
Diane on Cheers was an excellent bowler because she had taken bowling for her PE credits in college.
Quantum Leap's Dr Sam Beckett has seven degrees (most of them unidentified) and knows eleven languages. On the other hand, the Time Travel process leaves his memory swiss-cheesed, so he can forget as well as remember things as needed by the plot. Also, any skill he doesn't have can be taught to him by Al, who has been an astronaut, a boxer, a stage actor, a baseball pitcher, and even teaches Sam how to out-draw an old West gunslinger. In fairness, if you were looking for someone to be a holographic observer you'd probably want to get someone who is pretty handy, especially since they can't physically help. And Al is clearly old enough to have been all these things.
- Al has also been married five times, and when information he gives Sam seems too arcane for even Al to have personally experienced, he often claims he learned it from one of his ex-wives.
- There was an episode where Sam, in a life-threatening situation two switchblade-wielding kidnappers/rapists, was able to fend off his attackers WITH KUNG FU! After high kicking one of the scumbags, Sam was surprised to learn that he could do that, immediately after which Al informed him that he was skilled in several schools of martial arts.
  - If you think his skill in martial arts surprised us, imagine how the people around him (who see Sam as their friend/family member/whatever) felt!

Kevin: Mom... How?
Sam: Girl Scouts.
Kevin: Awesome.

—from the episode "Another Mother"

It's even in reality TV: In the Survivor All Star episodes, shortly after Richard Hatch gets voted off, we find out that Lex has a talent at fishing (which we never knew from Survivor Africa, because the only water Lex encountered there was at the bottom of a muddy hole).
In Lost, several characters have developed useful skills that have proven critical on the Island. Sun's knowledge of the medicinal qualities of herbs found on the island exceeds those of a normal home gardener, and Kate's "tracking" skills came out of nowhere (explained by a You Didn't Ask). However, this may be still explained by the mysteries of the island.
Shepherd Book of Firefly displayed an array of crime-related talents and knowledge that one wouldn't expect the average priest to have, hinting at a Mysterious Past; however, it was never made quite clear what this past was, beyond being former Alliance and his survival being very important to the authorities (except possibly non-canon Serenity book tie-in, where he reveals to Mal that he is a former Operative).
- The Serenity RPG has characters buy their skills as they go, the justification being that we find some new facet of the character's backstory at that point.
- Word of God has said that's canon now, as "Book" is actually the name of someone he killed.
On Friends, Chandler has hidden from Monica that he's an excellent ping-pong player, because he didn't want her to enter them in doubles tournaments. When she hurts her hand during a game, he turns out to be a not-so-hopeless replacement.
- This was hinted several times early on, with Chandler mentioning playing racquet ball with his bosses, or Ross' parents. This was probably something written in, because of Matthew Perry's real-life brief career as a tennis player.

Chandler: I'll just play left-handed.
Ross: You mean you're not left-handed?

Sandra Bennet of Heroes can make fake IDs, a skill she used to sneak into Def Leppard concerts when she was younger.
In the Bones episode "The Woman In The Sand," the normally clipped and clinical Brennan unexpectedly busts out a scarily convincing alternate persona to fool some lowlifes, complete with Jersey-girl accent. When Booth later asks her "what got into you," she says, "It’s from when I used to watch old movies with my dad. He really liked Clara Bow."

"Clara Bow was a silent film star, Bones."
"Yeah, but I guess that's how I always imagined she sounded."

- Possible Genius Bonus; Clara Bow was a Brooklyn native and had a notoriously thick accent.
In The A-Team episode "The Maltese Cow," Face and Murdock find a message written on a mirror in Chinese. Murdock proceeds to read it out loud, and when Face gives him a weird look, he explains:

Murdock: One afternoon, I got a gonzo headache, and before it was over, I could read and write Chinese. And that was a bad afternoon, let me tell you.

Trick's Ueda Jiro, virgin physicist professor, took a correspondence course in karate that somehow turned him into a kung-fu machine.
In Taxi, Jim Ignatowski is brought to a fancy party by Elaine who is hoping to impress someone but needs a date to attend the party. Jim proceeds to humiliate Elaine by acting like the '60s burn-out that he is, only to save the situation by sitting down at the piano and playing brilliantly. The other guests assume that Jim was just having a bit of fun, and so Elaine succeeds in her goal. Jim's befuddled reaction to his own playing: "I must have taken.. music lessons..."
In the Made for TV Movie/Pilot Move Tag Team two pro wrestlers are drummed out of the business for refusing to take a dive and become cops. At the police academy one (played by Jesse Ventura) shows remarkable marksmanship. "Where did you learn that?" "Navy SEALs."
In FlashForward, Nicole has a plot-useful knowledge of Japanese because she was a military brat.

Tabletop Games

It's not uncommon for characters using kung fu in Feng Shui to claim that a particular ability their player bought with XP was something they knew how to do all along. As the book explains, "Action heroes pull this one all the time."
A "cinematic" Advantage proposed by S. John Ross for GURPS is Up To The Challenge, which says that, within limits, a cinematic character can Suddenly Have Always Had Skills, as long as he spends the Character Points for them later.
- The two main limits were 'cannot violate prior game continuity' (if you had a prior opportunity where you "should" have known that skill but didn't show it, you could not claim to have 'known it all along' unless it was something you could plausibly have learned in the interim between that scene and now) and 'shtick protection' (right of first refusal on invoking Up To The Challenge always goes to the party member who is primarily built on that field of knowledge. Example: if your group has a character who is established as a master linguist, you only get to Ass Pull sudden obscure language knowledge if the linguist's player chooses not to).
In Mutants and Masterminds characters with the Languages advantage know a number of languages equal to 2^(their advantage rank) (character's without it know 1 or 0). Characters with 2 or more ranks often don't define all their languages known at character creation, leaving the blank slots to be filled in mid-play. Further a Hero Point can be spent to gain any advantage, except those that give new ways to spend hero points, for a scene making. This means a character can learn a language then forget about it the next scene. Both are perfectly standard for the genre the system was primarily intended for.
- Alternately you can just spend three character points on buying the universal translator superpower with the limitation 'Only for languages previously learned; takes at least 1 scene to learn a new language' and spend the rest of the game speaking any language you feel like so long as you 'could' have learned it earlier. (As three character points would buy you fluency in 12 languages if spent the normal way, this is actually game-balanced and its first use was in published official material.)

Video Games

The computer game Planescape: Torment gives us The Nameless One, an amnesiac immortal (or is it immortal amnesiac?) who, rather than learning skills from people that he meets, instead remembers whatever skill it was that he forgot, that, occasionally, he taught the teacher ages ago. It goes a step further by having you relearn skills by re-attaching severed body parts that you had already grown back, which involves re-severing them. And one of them is your eye.
Gordon Freeman: the 27-year-old theoretical physicist who just happens to be a crack shot with all types of human and alien firearms, in addition to being able to expertly wield a crowbar as a weapon.
George of the Visual Novel Umineko no Naku Koro ni reveals he knows a multitude of martial arts while fighting Gaap in the fourth story arc. Granted, it was known as early as the start of the first arc that his mother Eva was a skilled martial artist, and that she taught George many, many things to become the talented and successful man he is. But because it wasn't known that martial arts was one of those "many things" until later, it still counts.
Metal Gear Solid 3: Snake Eater introduced CQC fighting techniques to the series. They were brought back for Metal Gear Solid 4: Guns of the Patriots, but in 3 you were playing as Big Boss while in 4 you were playing as Solid Snake. So, it was retconned that Solid Snake always knew CQC because Big Boss taught it to him. In one Codec, Otacon asks why Snake never used them before, and he says something about how using Big Boss's moves didn't "feel right" but now he's apparently okay with it, or something.
- Snake hadn't used CQC before because they were taught to him by a man he felt profoundly betrayed by. When Snake resolves that feeling of betrayal, he's willing to use Big Boss' gift to him again.

Western Animation

Hey Arnold!!, "Road Trip": See Broken Streak for details on Helga's mom riding bulls.
In one episode of Kim Possible, Dr. Drakken hypnotizes all the senior citizens in Florida, including Kim's Nana, who it turns out has had quite an interesting life and picked up an assortment of skills, including piloting, race-car driving, and of course, martial arts.
- Given that the Possible family is a clan of adventurers, that's hardly even surprising. Nana was simply the Kim Possible of her day.
Winx Club had fashion-obsessed Stella solve a puzzle in one episode by remembering a minor detail from a field trip they took for magical history class. (It should be noted that this is a dub change: in the original, it's her love of fashion that helps her solve the puzzle, although with this method she essentially stumbles on the solution.)
The Boondocks. Who knew Uncle Ruckus "had mastered the ancient and deadly art of the nunchaku".
In the Justice League episode "Hereafter", Superman awakens on a wasteland planet with a red sun, robbing him of his powers, with only a city street's worth of objects to assist him. As he attempts to find a way off, the human-level Kal-El displays shocking aptitude in blacksmithing, sword fighting, auto repair, animal husbandry and even dog sledding as he traverses the blasted landscape.　Many of these skills could have been picked up on the Kent farm where he was raised, or reasonablly extropolated from them. The source of his knowledge of sword fighting is perhaps an adventure onto itself.
In the most recent season of Futurama, it is revealed that Dr. Zoidberg's doctorate is in art history when the main characters need to analyze a Da Vinci painting.
One episode of Archer reveals that Pam, head of ISIS PR, was an underground bare-knuckle boxer to pay her way through college, thus making her an even better fighter than most of the field agents. She also apparently races against the Yakuza on occasion.
In one Tiny Toon Adventures episode, Buster and Babs rescue a baby beluga whale—and can communicate with it easily thanks to Babs, who in her own words "just happen(s) to be fluent in Whale".

Web Comics

In Harkovast, Chen-Chen at first seems pretty helpless, but is suddenly revealed to be a master of the sliding mountain style, which allows her to kill armed opponents with her bare fists.

Web Original

Played with in one Global Guardians story: a villain tries to hold inoffensive precog heroine Second Sight hostage in order to escape. Second Sight, whose powers all revolved around her clairvoyant ability to see the future, took the villain apart with previously unseen Kung Fu skills. It turned out the skills had been there since the creation of the character. They'd just never been needed before.

Real Life

When Uwe Boll challenged his many critics to a boxing match, most people thought it would be in good fun. It turns out that Boll was a semi-professional boxer before taking up direction. He beat the ever-loving snot out of several untrained critics who went into the ring with him. It should be noted, however, that Boll openly cherry picked his opponents for lack of fighting experience and size and repeatedly refused challenges from critic Seanbaby, who knows Muay Thai and is much larger.
While filming a scene where a character is stabbed in the back, Christopher Lee explained to Peter Jackson that when that happens, the victim doesn't yell. When asked how he knew, he explained that during WWII, he was in the British Special Operations Executive (their equivalent of the OSS). The story is told in the extras for the The Lord of the Rings and you can watch it here.
This crops up a lot in countries where compulsive military conscription is the law or was until recently. For example the nice middle aged gentleman might have once been Spetsnaz, the fellow walking down the street might have been a medical corpsman or your disgruntled worker might have set claymore mines as a combat engineer...