In Alice in Wonderland (2010 version), Alice spends almost the entire movie running and hiding from things more powerful than her. At least, up until the climax, where she starts reciting her Six Impossible Things Before Breakfast. The last Impossible Thing is "I can slay the Jabberwocky", and once she says it she finally starts attacking and fighting the beast off.
In another example, the formerly comedic and apparently useless other policemen reveal themselves to be competent badasses.
In the oddest and funniest use of this, the evil schemers of the town turn out to all be members of the Neighbourhood Watch Association - and almost all of them are over 70. This includes the Police Chief, a woman who runs a corner shop, a husband and wife who run a pub, and a priest! And they're all fucking badass!!!
This is the entire point of Sky High. All the quirky "sidekick" characters use their powers to great effect at the end. Granted, one of them simply refused to take the classification test and didn't use her full powers until the big fight...
Reservoir Dogs has the dying Mr. Orange, who before appeared to be a sniveling wreck (not to mention an unconscious one) dragging himself up and shooting Mr. Blonde. Fifteen times. And then repeatedly pulling the trigger of his empty weapon, just in case there was another bullet hiding in there somewhere. Blonde, having finished torturing the police officer, was about to light him on fire. Orange wasn't going to let that happen.
Even though we knew Yoda was supposed to be a badass, really, his ping-pong lightsabering in Attack of the Clones was still totally unexpected. Yoda also does this before/after in The Empire Strikes Back. At first, he appears to be a harmless yahoo who might conceivably be useful in guiding Luke to someone else, and a few scenes later he's lifting Luke's fighter out of the bog. With his mind. Granted, the argument with the ghost of Obi-Wan is a pretty big hint, but still, the first time you see it, it's an eye opener. And this is taking into account the fact that Yoda's implied to be a bit rusty/out of practice by this point, what with having been hiding from Palpatine this whole time. ...And that he's in the final year of his 900+ year existence.
The Ewoks. Despite their size, they are still a warrior/hunting race with old-style ingenuity.
The three masters of Pig Sty Alley kick the crap out of the Axe Gang after looking pretty ridiculous earlier on.
The Landlord and Landlady display their superior abilities against the zither-playing assassins after looking like little more than a trashy, middle-aged couple.
The Beast is a bald, tubby old man in a wife beater, boxers, and flip-flops, but displays his Bad Ass abilities by casually catching a bullet with two fingers, fired inches away from his head.
Sing spends most of the film as the Butt Monkey, but transforms into a one-man wrecking crew for the final battle.
Snakes on a Plane: "Enough is enough!" I have had it with these m*f* snakes on this m*f* plane! And the rest is history. Not that they weren't getting dangerous before that. But this is when the movie completely acknowledges what lengthsthey'll go to.
Shaolin Soccer uses the same trope. The team of monks play their first game with a local bunch of hooligans and are getting their butts literally kicked until they turn themselves around and finally harness their kung fu superpowers.
Captain Chaos from Cannonball Run. "When he puts on that cape and that mask, he'll blow your goddamed doors off!'
In Star Trek (2009 movie), Sulu pulls this off. After several lighter, comedic moments, he proceeds to brawl out two massive Romulans (while Kirk is busy getting his ass kicked, no less).
The Captain asks for volunteers with "Combat Training". In the shuttle, Kirk asks Sulu what his training was: the response, "Fencing", gets a look of disdain from Kirk. Landing on the drill, Sulu promptly whips out a folding sword and goes to work
The Ragtag Bunch of Misfits that end up joining with Shua and Dr. Noah in Sky Blue display considerable cunning and fighting skill when they attack Ecoban.
Richard, Jennifer Aniston's brother in The Breakup, played by John Michael Higgins. Though a Flamboyant Gay, Richard delivers a quick curb stomp when Vince Vaughn lays a hand on him. Later, the ego-hurt Vaughn complains that he was sucker-punched before he knew he was in a fight, and promises to bring some "Polacks who don't have a future" to back him up next time.
In Transformers: Revenge of the Fallen, the aging Decepticon Jetfire seems to be a harmless, senile old robot that spends his days hiding in a museum. That is, until the last battle, where he flies in, whips out a battle ax, and cuts another robotic combatant in half while yelling "Let me show you how we brought the pain in my day!"
Who Framed Roger Rabbit: When Eddie Valiant, who's already been starting to drop the alcoholic slob act as he's been forced into action, gets ready to finally enter Toontown, digs out his giant toon gun, and instead of taking a swig from his bottle of booze, throws it in the air and shoots it with an anthropomorphic Indian bullet that produces a tomahawk bigger than itself and smashes the bottle in mid-air. That's the only thing the gun ever is useful for in the movie.
Roger himself also gets a moment to shine. After Eddie and Jessica are taken captive, Roger attempts to save them by bursting up through a drain in the floor. (Hey, he's a 'Toon. He can do things like that.) In a Crowning Moment Of Awesome, he holds Judge Doom at gunpoint and declares "We 'Toons may act idiotic, but we aren't stupid!" Of course, his heroic act comes to a halt after a literal ton of bricks is dropped on his head.
Mr. Miyagi in every single The Karate Kid movie. Given, it's no longer surprising after the first movie, but it isn't until the end of the first film that he demonstrates he "still has it after all those years."
In the Russian film Guest from the Future, a space pirate corners the child protagonists. Then their sports teacher shows up, the pirate threatens her, she throws him out of the window — on the 5th floor...
Also, in Dead Man's Chest, Pintel and Ragetti, who have up to this point been portrayed as bumbling idiots, are seen carving up some of the crew of the Flying Dutchman, who are nigh-unkillable supernatural baddies.
Jack Sparrow himself is eccentric, never fights when he can run or swindle an generally acts a coward. Doesn't stop him from being one of the best swordsmen in the franchise.
Anna in Hudson Hawk is more or less useless throughout the movie, due to being a nun, until of course Minerva Mayflower makes the mistake of insulting the Big Guy.
Patrick Winslow and the time-lost Smurfs approach Gargamel's hideout in Central Park to rescue Papa Smurf, with Patrick wielding a crowbar, the Smurfs cricking their necks, and the soundtrack playing AC/DC's "Back In Black" in The Smurfs.
A similar scene plays out in The Smurfs 2 as Patrick Winslow goes to help rescue Smurfette with him wielding a fire poker.
Mané Galinha in the Brazilian movie City of God. He was just the local Casanova, until Zé Pequeno utterly humiliated him (even forcing him to watch his girlfriend being raped) and attempted to kill him, only to kill his little bro instead. Mané Galinha picks a gun and alone chases off Zé's gang, killing some and scaring the shit of the rest. He later becomes the Fallen Hero.
The schoolboys-turned-cowpokes in The Cowboys. The rustlers caught them off-guard and by surprise at first, but then the bad guys went and shot John Wayne. In the back. At that point, the villains became dead men... they just didn't know it. And the cattle rustlers certainly didn't expect a bunch of 12-year-olds to take them out.
Randy Owens in the 1957 movie Designing Woman is introduced as an effete dancer, but, at the end of the movie uses his dancing moves to knock out a bunch of thugs.
Private Hudson in Aliens. The film's Plucky Comic Relief, he spends most of the film spouting one-liners and Badass Boasts or whining and complaining when the mission inevitably goes bad. Eventually, however, he ends up going full-on berserk at a horde of Xenomorphs and attempts a a You Shall Not Pass that buys the heroes time to escape.
In the 1939 WesternDestry Rides Again, the eponymous character (played by Jimmy Stewart in his finest "nice guy" style) becomes the deputy of the marshal in a lawless town. At first, he seems to be completely harmless; he doesn't carry a gun, drinks milk at the bar, tells anecdotes to everybody, and makes napkin rings as a hobby. However, he eventually proves to be a very capable gunfighter.
We're the Millers: Near the end of the movie, Dan disarms and subdues a Mexican drug cartel enforcer using nothing but a mug.
Johnathan, the Plucky Comic Relief from The Mummy Trilogy. The guy's a greedy, cowardly swindler, but he later demonstrates devastating accuracy with a rifle, good pickpocket skills, and disturbing proficiency with a tome of magic spells.
Faisil Heslov, the geeky junior agent in True Lies. He's skinny, inexperienced, geeky, and absolutely the type of character who usually ends up a Sacrificial Lamb. He ends up taking out two of the terrorists by himself in a smooth bit of gunplay that ends with him blowing across the end of his gun, James Bond style after walking into the terrorist's den disguised as a TV cameraman.