This is the central theme of many, many kung-fu movies (such as Drunken Master and Fearless Hyena), in which the central character learns a new style of martial arts in order to defeat the villain. The American equivalent is the original The Karate Kid.
In Mulan, the "I'll Make a Man Out of You" sequence is a perfect example of this happening to an entire army.
Oldboy lampshades this. The philandering, perpetually-drunk thirty-year-old salaryman spends 14 years in a private prison trying to train his body to be tough. When he gets out, he wonders if all that training paid off when he gets into a fight. It turns out, it did.
Accomplishing this may, in fact, be the central thesis of Wanted (Whereas the comic was more about supervillains just being supervillains).
The trope is played very straight in Jason X, in which an android girl is given an upgrade that makes her an instant commando. She subsequently blows Jason up with a BFG.
Clarice throughout the Hannibal series. Starts somewhat badass and comes out crazy-scary.
Between the original 80's cartoon and the 2007 film, Starscream has taken a major level in badass. To prove it, we have this line as Starscream flies over Mission City, from frigging Ironhide, of all people:
The human character, Sam Witwicky, who spent the first two movies running away from these giant robots that try to blow him up or squish him (Hey you would too), suddenly has clearly had it with everything, and seriously invoked this trope. How Bad Ass has Sam become? Well if you must know... HE KILLED STARSCREAM!
He even levels up his running away, having learned Parkour at some point between the second and third movies.
Megatron is pretty infamous for his Badass Decay. He spends most of the third movie as an injured, sulking cripple until Sam's love interest Carly, of all people, rouses him into beating the pulp out of the Bigger Bad, and he does a fine job indeed.All while still sustaining his injuries. To be fair, he shoots Sentinel Prime in the back, while the latter is busy beating up Optimus. This, however, gives Optimus the chance to decapitate Megatron and blow Sentinel's head off in the space of a few seconds.
This pales in comparison to what he does in Transformers: Age of Extinction - he manages to stay alive as a severed head, and manipulates the apparent human Big Bad's faction into building him a new body. Now Galvatron, he possesses a Nigh Invulnerable body and nearly overpowers Optimus in one-on-one combat - all while posing as a remote-controlled drone - until Lockdown interrupts their fight. He then builds himself a new Decepticon army using all the humans' other drone Transformers, and even though he's largely Saved for the Sequel, he still comes across as far more menacing and competent than in the two previous movies (and maybe even the first).
Hell, just like Starscream, Megatron has taken a level in badass between Transformers Generation 1 and the first movie (maybe even the second too). Gone is the General Failure incompetent cartoon villain commander (see the Western Animation section for more on that). This Megatron is a demonic-looking mechanical beast voiced by Hugo Weaving who rips Autobots in two, kills Jazz and Optimus, treats humans like insects and doesn't hesitate to flick those pesky buggers out of the way, and is absolutely ruthless in pursuing his objectives. And if supplemental material is to be believed, he's a cannibal too.
NEST infantry, by the third film, managed to pull out an unqualified victory in an engagement with 'Cons, instead of assisting the Autobots. In fact, the 'Bots show up to assist them.
Pasha Antipov, a minor character in Doctor Zhivago, begins the movie as an utter wimp. He's a cuckold, a bespectacled naif, and an ineffective revolutionary. Even his name is a bit puny. But then he's hit by a bomb on the Eastern Front and drops out of the movie—only to return in an impossibly cool scene as the enigmatic "General Strelnikov," a Badass Longcoat with his own armored train.
In the second Pirates of the Caribbean movie, delicate but spirited Damsel in Distress Elizabeth Swann has become a quite competent sword-wielding Action Girl. This is handwaved in a throwaway line: her fiance Will Turner, the best swordsman in the series, has been teaching her for the past year. Then again, she is hanging around with pirates... Even more so when she becomes the captain of a ship, escapes from Davy Jones, and then becomes the Pirate King (King, not Queen). Will was already somewhat of a badass, but becoming the captain of the Flying Dutchman certainly counts.
In The Chronicles of Narnia, the dryads go from sissy flower ladies to deadly trees that destroy most of the Telmarine army. Likewise, Lucy Pevensie. In Lion... she does essentially nothing. In Prince Caspian she does no more than show off her dagger with Aslan as backup standing right behind her. By the time Dawn Treader rolls around, she's throwing herself fully into close-quarter combat and beating her pirate captors unconscious with blunt objects. Granted, older sister Susan gets a similar on-screen moment of glory in Prince Caspian, but it was always hinted (but never shown) that she'd done her bit at the battle at the end of the first film.
One of the most jarring examples would be Sarah Connor's transformation between the first two movies: She (and the actress) was a normal, happy-looking girl in the first one, but looks like she lost about 30 pounds of body fat and transformed the rest into solid muscle by the second one, as well as going from Damsel in Distress to Psycho Action Girl. A decade of preparing for The End of the World as We Know It will do that to a person. Special mention goes to the scene at the end of the movie where she, if not for running out of ammo, would have destroyed the T-1000 on her own without the T-800's help. BAD. ASS. Special mention has to be given to the actress, Linda Hamilton, who in preparation for the role trained for 13 weeks under a former Israeli commando, undertook strenuous weight training, and learned how to pick locks.
John Connor upgrades himself from fairly Wangst-ridden drifter who specializes in running away from Terminators who reluctantly gives vague radio support in Terminator 3 to Colonel Badass able to rewire AI motorbikes and shout down experienced Generals and killer androids by Salvation. Quite a few levels in badassery and self-confidence gained it would seem.
Another alarming one was Straw Dogs, in which Dustin Hoffman starts out as, well, Dustin Hoffman, in the role of a mathematician. As the local louts gradually cause him more and more grief, however, he goes a bit mad, and ends up brutally murdering them all when they try to break into his house.
As a general rule, the less someone did this in The Descent, the faster she died.
In Feast, when all the hero types are dead and hope is lost, the mother who lost her son (Tuffy) "levels up" and becomes Heroine 2. Then she proceeds to punch all the monsters teeth out with the butt of her gun and punch its stomach through the mouth, choking it to death. Crowning Moments of Awesome? Yes, you could say that. In Feast 2 she spends much of the movie surviving, but surviving through a Feast movie is a pretty big deal.
In The People Under the Stairs, Fool was originally reluctant and a tad wimpy to go in the house is seemingly like an average kid, but when Leroy is killed and is chased several times, he willingly takes on anything that comes in his way, such as punching a dog in the face, later killing him holding a villain at gun point, punching him in the nuts and then blows the house up killing the villains, saving the day in the process.
Uhura takes multiple levels between the series proper and the films. While her snark - and the Mirror Universe episodes - have always hinted that she's far more than a Bridge Bunny, the films stop hinting pretty quickly.
Luke Skywalker may have set the record for most levels in Bad Ass taken up in a single trilogy. He's a naive, whiny teenager who complains about having to clean droids at the beginning of the first movie and becomes the man who rescues a princess, nukes a Death Star, becomes an ace pilot and a celebrated war hero, amputates a number of bad guys, takes out a giant Walker with a hand grenade, survives an endless fall with the use of only one hand, gets into several lightsaber duels, rescues his friends from Jabba the Hutt and Boba Fett, gives the Emperor the finger, and brings his dad back to the Light.
Anakin Skywalker takes a level in badass between Episodes II and III, becoming a full Jedi Knight (and losing the awful rattail). By the time of the original trilogy this has flip-flopped though, with him being more ruthless and physically stronger as a cyborg but less powerful with the Force (and not to mention horribly crippled).
Sgt. Powell in the first Die Hard. Munching donuts and relegated to desk duty in the beginning his gets a huge Crowning Moment Of Awesome at the end when he shoots the final bad guy dead in the face complete with the Rousing Music playing in the background.
Evil Dead 2: Ash starts out as a wimpy college kid, but eventually cuts his own hand off and arms himself with a chainsaw and sawed-off shotgun, remarking, "Groovy!"
Abe Sapien between the first and second film. He goes from the geeky, psychic Non-Action Guy to someone that carries a gun and has enough martial arts ability to at least evade and stall a troll three times his size, if not actually damaging it. Still geeky and psychic, but a bit more power behind it.
Liz Sherman even more so. She goes from a scared, childlike woman who has no control over her powers to a fearless, SWAT-gear-wearing, room-torching, sharpshooting badass in the second. Being a BPRD agent is incredibly Darwinistic; you either become a badass or you die.
Done in Paul Blart: Mall Cop where Paul initially manages to defeat the Le ParkourTotally Radical mall robbers through large amounts of dumb luck, upon learning that his daughter is amongst the hostages he takes a massive level in badass, setting up a fair amount of traps and managing to stealthily take out the remaining goons with a combination of disguises and using his location (IE getting them to come to the Rainforest Cafe and hiding amongst the animatronics).
Johnny 5 went this way in Short Circuit 2. After being nearly destroyed, he rebuilds himself as a crazy-ass punker robot and goes on a rampage.
Between the second and third films, Doc Brown (of all characters) takes a level in badass. The third film sees him toting a big bad rifle, saving (and subsequently wooing) a damsel in distress, standing up to the local gunslinger, hijacking a train and driving it off a cliff (so it can hit 88 miles per hour and travel in time, not to commit suicide).
George McFly. With an assist in the past from his as-yet-unborn child, he goes from a mousy, cowardly junior-level worker to a highly successful writer... and along the way, Mrs. McFly gets a little spillover badassery (or at least some weight loss and athletic skills).
Jason Tripitikas takes several levels in badass in The Forbidden Kingdom after he trained with Lu Yan and the Silent Monk. It isn't enough for him to take on The Dragon (Ni Chang), but it's still a vast improvement. Then again, it's Jackie Chan (Lu Yan) and Jet Li (Silent Monk), the two of them combined could grant a 90 year-old grandmother several levels in badass.
In Moonwalker, Michael Jackson mostly does shit like running through a field of flowers with some children. However, at one point, he takes a level in badass by grabbing a Tommygun and mowing down a bunch of ghetto stormtroopers. Toward the end, he takes another level in badass and turns into a giant robot and blows the shit out of a bad guy's evil lair.
Rocky Balboa, the titular character of the Rocky series of films must always have a mandatory montage in the films where he trains HARD. Pushing the absolute limits of his body, will, and expectations of him...these montages always end up with Rocky gaining a new "power" and thus...a new level of his already badass status. You can even visually see his badass level go up. Set to the song Gonna Fly Now, this Training Montage is one of the most famous film conventions of the modern age.
In the 1st he gets the dedication and conditioning necessary to "go the distance" with Apollo Creed.
The 2nd sees Rocky gain some speed and the ability to switch his dominant fighting hand.
3rd gives Rocky a tremendous boost in speed and offense.
The 4th gives Rocky the ability to show Russia just how awesome he is by training in the snow by pulling chains.
The 6th (we'll skip the 5th) sees an aged Rocky gain overwhelming power to compensate for his eroded speed due to age.
Over the course of District 9, Wikus from goes from sweater vest wearing pansy to badass who single-handedly breaks out of a fortified government facility and then later chooses to break back in.
Daphne from Scooby-Doo mentions how she is sick of being the Damsel in Distress and thus takes kung fu lessons to defend herself. This pays off when she defeats a masked wrestler guy, even taunting him. "Now who's the Damsel in Distress?"
Morgan Sullivan from Cypher starts out the movie as a timid, cuckolded house husband who's looking to make his life a bit more interesting by becoming a corporate spy. By the end of the movie he's sat through an intensive brainwashing session without batting an eyelid, escaped from an ultra-high security data centre and blown up a small army of Mooks. Subverted in the sense that he's been a badass all along and didn't know it.
During Firefly River Tam spent most of the action scenes she is involved passively crying or running away (with a couple of notable exceptions that she might not have quite registered as fights in the first place "No power in the 'verse can stop me."). In the movie Serenity, however, she is quickly upgraded to a killing machine who defines Waif-Fu, and can take out roomfuls of armed assailants without breaking a sweat — sometimes brainwashing has awesome results.
In Enchanted, Giselle goes from helpless Disney Princess to climbing a building and slicing a dragon's tail with a sword.
In the Laurel and Hardy film Way Out West, Ollie becomes surprisingly forceful and effective at movie's end in getting the deed to a gold mine back to its rightful owner away from the couple who conned it from them earlier.
In The Princess Bride, Westley goes from a poor farm boy to Dread Pirate Roberts level of badass, immune to iocane powder.
Angelica Chaste, in Angels Dance, starts out as a repressed mortician who thinks a doll is her baby. When she finds herself the target of an aspiring hitman learning the trade by assassinating a randomly picked individual she gets scared...then she gets mad. By the end of the movie, she's levelled-up in Badass to the point that she's as much of a threat to her pursuers as they ever were to her.
Even before then. Just after the sacking of Severus Snape, when Harry asks that the professors buy him some time and he goes off in search of Rowena Ravenclaw's diadem, its Neville that Harry puts in charge in his stead with a casual, "Hold the fort, Neville." To his credit, Neville doesn't even flinch. He just nods in acceptance of this duty.
In the first Mummy film, Evie is a timid librarian firmly stuck in her Damsel in Distress role. By The Mummy Returns, she has become an Action Girl who can fight off Mooks on her own using awesome martial arts skills that she didn't know she had, as well as a few of Rick's more straightforward moves. It turns out she is rediscovering her Secret Legacy as a spiritual reincarnation of an Nefertiti who was well-trained in combat. Her brother Jonathan too, to an extent. He is a pretty good shot with a rifle and proves it during the ambush of the cultists. He also tries to take on Anuksunamun and manages to hold her off long enough for Alex to revive his mother.
Godzilla Junior in the second Godzilla series. During his first two appearances in Godzilla vs. Mechagodzilla II and Godzilla Vs Spacegodzilla, he was much like his Minilla, and was actively disliked for it. Then along comes Godzilla vs. Destoroyah and Junior has become a badass Teenzilla, who despite being a Gentle Giant and overpowered by the titular villain, hands Destoroyah's penultimate form its ass after a long, brutal fight where he gets to demonstrate just how much of a determinator he is. Destoroyah's freaked out enough that after it's resurrection into its final form it goes after Junior first, despite the adult Godzilla having arrived. Temporarily killed, he is resurrected by Godzilla's death as the new Godzilla and, assuming that he is the Godzilla who appears in Final Wars, may now be even more badass than his dad.
The protagonist in Layer Cake spends the first half of the film thinking he's absolutely on top of his game; he's repeatedly proven wrong when people turn out to have been plotting against him and playing him, resulting in his life spiralling out of control. After he decides to kill his boss, who has been siphoning off his money and is about to rat him out to the police, he becomes much more ruthlessly efficient and starts setting up gambits of his own.
And he takes another level in the sequel. Perhaps more.
In Up, Carl Fredericksen is a 78-years grumpy, sour, etc., old man who is completely obsessed by his house, which he considers as the last remnant of his late wife, who he feels he "betrayed". He even goes as far as to let the Big Bad capture a rare bird he earlier swore to protect to save the same house (and gets a "What the Hell, Hero?"). But when a Crowning Moment of Heartwarming makes him realize at last that the house is nothing but a house, that his memories are what are keeping his wife alive within him, and that he never "betrayed" her, he immediately goes from old grumpy selfish almost-Jerkass to happy and brave Badass Grandpa, recovers the energy and strength of his youth, and sacrifices the remnants of his house to be able to go and save the bird.Never underestimate The Power of Love!
Megamind: Megamind was half-joke in the beginning of the movie: a super villain who always loses, can't even impress his lady hostage with his instruments of doom anymore, and isn't really evil so much as really enjoying the endless game of being the villain to an invincible hero like Metroman. But look at him in the final battle! He's not completely different - even during the battle he's hilarious - but he pulls off some really cool moves, especially for the climax of the movie.
How to Train Your Dragon: Hiccup starts as a weak workshop assistant, and levels up until he's an aerial combat tactician able to take on a dragon the size of a skyscraper and win.
The character Johnny English is a secret agent who can be best described as a Crouching Moron, Hidden Badass...with a considerable emphasis on "moron". After a disastrous mission in Mozambique, English is sent to Tibet to undergo a special training. Not only did he take a level in badass, he acquired Balls of Steel as well.
Loki seems to have taken a few—possibly traumatic—levels in badass between the events of Thor and The Avengers. While he was ostensibly the main antagonist in the former, his return in the latter is accompanied by a shiny new spear of doom, an army of alien cyborgs, and a new plan to rule the Earth. The scariest part is that he seems pretty capable of carrying out this plan.
Hawkeye and Black Widow start The Avengers as assassins and secret agents, but by the end they're straight up superheroes able to keep up with gods, monsters, and super soldiers.
Literally forms the basis of Captain America: The First Avenger's plot, with courageous but scrawny Brooklyn kid Steve Rogers being transformed into the ultimate super-soldier: Captain America. It also emphasizes the fact that he still manages to be the same Nice Guy that he was before the super-soldier procedures when he's transformed.
After being kidnapped by Thomas, her stalker, and failing to call the police or escape from the parking garage in P2, Angela is driven to knock out the security cameras with an axe, escape from a locked trunk with only a crowbar, kill her kidnapper's dog with said crowbar, and steal a car and end up in a game of chicken. This isn't even counting her Playing Possum, cuffing Thomas to the car, and finally setting him on fire with gasoline and a taser when he calls her a cunt.
In Tangled, Rapunzel starts out as a sweet but incredibly naive girl who lives in fear of breaking her darling "mother's" rules. At the end of the movie, when she confronts Mother Gothel about kidnapping her as a baby, Rapunzel stands up for herself and coldly says that Mother Gothel will never take advantage of her again, shoving Gothel away when she tries to rein Rapunzel in. And when Gothel stabs Flynn and tries to kidnap Rapunzel away again, Rapunzel sets up a huge resistance, saying that she will never stop trying to escape unless Gothel lets her heal Flynn.
As in the books, Gandalf the Gray from The Lord of the Rings, already quite a powerful wizard, dies in his fight with the Balrog, but is resurrected and comes back as Gandalf the White - master wizard, great warrior and all-round ass-kicker extraordinaire.
North and South: The 4 main male characters definitely. They start off as dapper gentleman or scruffy goof-offs, but eventually work their way to becoming quite tough and capable, most notably in Charles who is more than badass enough to handle any number of assailants in hand-to-hand combat. Some of the ladies also show remarkable toughness. Brett yanks her own sister off a horse and threatens her with a pitchfork when she tries to get Billy executed. Semiramis comes within an inch of murdering Salem Jones. Madeline attacks Justin with a saber when he refuses to let her go.
Happy Hogan, of all people. After Ivan Vanko makes his grand Dynamic Entry during the race that Tony forces himself into, and is about to make his finishing blow on Tony, the bodyguard, in Tony's custom limo, comes out of nowhere, rams into Vanko full force, and manages to keep him effectively pinned against the chain-link fence for several minutes. He's shown to be decent in a fist fight but not on Black Widow's level.
The Mk II goes from a flight test beta to the embodiment of More Dakka. As discussed by Rhodey, this was the point.
Dredd. Anderson starts out as a rookie cop in over her head, and becomes a full on badass. Dredd himself even notes the change.
At the beginning:
Dredd: You ready rookie?
Anderson: Yes sir.
Dredd: You don't look ready.
And at the end:
Dredd: You ready?
Dredd:(approving) You look ready.
Kitai's character-arc in After Earth is strongly about this.
In the second Sherlock Holmes film, Mary Watson definitely achieves this during the scene on the train and afterwards when she is pushed into the river by Holmes to save her, remaining remarkably calm for someone who just survived such a fall!
In comparison to X-Men: First Class where her only physical scenes involve weight training and her shape-changing abilities used only as a distraction to aid someone else, Mystique has developed into using some of the acrobatic fighting style that her older counterpart from the first trilogy excels at.
In X-Men: First Class, Havok lacks accuracy with his destructive powers, but shows signs of improvement by the end. In his brief scene here, he neutralizes a soldier using a non-lethal blast with little effort.