Becoming the Boast

Bob brags about some skill he possess, or some impressive feat he pulled off in the past. In fact, Bob brags quite a lot about this. Alice may roll her eyes at the time, but she remembers. Because, inevitably, Bob and Alice will find themselves in a situation where success (and possibly survival) utterly depends on that skill Bob has been bragging about. At which point Bob will confess, with great embarrassment, that his original claims were very overstated, if not completely fabricated. However, while Bob's first instinct is to turn tail and run, that is simply not an option: by the time Bob is exposed, Alice's plan is completely past the Point of No Return. Since Bob is the Closest Thing We Got, he has no choice but to attempt to live up to the original boasts.

Naturally, since it's a Million-to-One Chance, Bob does just fine.

If Bob does turn tail and run, then this trope is not in effect and Bob's just a Miles Gloriosus. Not to be confused with Becoming the Mask or Fake Real Turn.

Anime and Manga

 * In Eyeshield 21, Hiruma likes to brag that he has a kicker on his team who can score a field goal at 60 yards, over half the field away from the goalpost, calling it the 60 Yard Magnum.  What follows is one of the most beautiful and kick ass scenes in the entire series.
 * Hiruma forced Sena into the identity of "Eyeshield 21", the ace running back from Notre Dame High School, so he wouldn't be scouted by other sports teams and Mamori wouldn't flip about him playing such a dangerous sport. As Sena later discovers,

Film - Live Action

 * Three Amigos: Ned mentions that a passing biplane is the same kind that he flew in one of his prior movies. At the end, the other Amigos need him to fly this plane to escape from El Guapo's army. Ned confesses that it was his stunt double who actually flew the plane. They climb aboard the plane anyway, and Ned flies like crazy.
 * In Memphis Belle bombardier Val has not exactly gone out of his way to deny that he is almost qualified as a doctor, even if he never specifically claims so. When Danny is wounded during the mission and the others look to Val to save him, he finally fesses up, admitting he only took two weeks of medical school before enlisting. The Captain, Dennis, gives him a speech about how he's the closest thing they got, and he goes off to save Danny's life...
 * In Snakes on a Plane, Troy claims to be able to fly a plane, and have logged hundreds of flight hours. And then it turns out that actually, he was talking about a PS2 game, but he manages to land the plane anyway.
 * Faintly justified; some flight simulator games, notably Microsoft's Flight Simulator, are accurate enough to be used in training real pilots.
 * In fact, as Mythbusters showed,landings can be somewhat automated on modern commercial planes assuming there is no damage to systems or especially bad conditions. A flight controller was able to talk Jaime through a plane landing and then showed an easier way it could be accomplished.
 * Mystery Men: Mr. Furious supposedly has the superpower of rage-induced super-strength, but, when called on it near the end of the film, he reveals it's all an act. However, when rescuing the Love Interest from the clutches of the Big Bad, he becomes genuinely furious, genuinely gains rage-induced super-strength, and starts kicking ass.

Film - Animated

 * Chicken Run: Fowler talks constantly about the time he spent in the Royal Air Force. At the end, the chickens expect him to pilot their homemade aircraft to freedom, and he admits that he was in the Air Force as a mascot, not a pilot. He ends up flying anyway. Interestingly, he's not the least bit ashamed at being a mascot, and seems surprised that the others actually expected him to be able to fly a plane. I mean, he IS a chicken.
 * The title character of Rango finds himself boasting big about having killed several men with a single bullet and, after accidentally accomplishing the feat, being made sheriff, at which point he has to do it for real.

Literature

 * A Star Trek: Deep Space Nine novel once featured Sisko bragging about being able to hit a target with a phaser blast using a mirror. Later, that exact situation crops up on the station (with Odo becoming the mirror); Sisko pulls the stunt off but afterwards admits he had exaggerated his ability.
 * Ciaphas Cain is constantly on the receiving end of this because of his (partly accidental) status of a Fake Ultimate Hero. The best example probably being in the first novel, when his friend assumed he'd want to sneak into an enemy camp for fun (although he is quite competent he's also an admitted Dirty Coward).
 * In the Septimus Heap series, a ghost asks Septimus if he can perform a certain spell and he says "almost"—not because he knows anything about the spell, but because he knows the ghost will get horribly upset if the answer is "no". Of course, later on they need the spell to heal a dragon-boat and they scramble to make it work.
 * Subverted in Harry Potter and the Chamber of Secrets. Gilderoy Lockhart's an amazing fighter of dark-forces- werewolves, hags, vampires, he's fought them all and managed to write about them. Except in reality he's an inept fool; pretty much everyone realises this and their suggestions about Becoming the Boast are mostly out a joy to see him squirm.

Live-Action TV

 * An early episode of Star Trek: Deep Space Nine has a Bajoran who is famed as being a great resistance leader, who later admits he is nothing of the sort. It's something of an inversion in this case, in that his reputation was not his own doing (he was, in fact very uncomfortable with it), but actually spread by other Bajoran freedom fighters who needed a hero to inspire them. He did actually kill an infamous villain in a one-on-one fight, but it was mostly a lucky accident. After that, as his legend grew, he got credit for almost every successful campaign by the resistance despite never having actually been in charge or heavily involved in carrying them out. But when the time comes he shows that he can be every bit the legend that people already believe.
 * In an episode of Seinfeld, George claimed to be a Marine Biologist to impress a woman. Later, he and that woman were walking along a beach when they came upon a crowd gathered around a beached whale who is having trouble breathing. George removes a golf ball from the whale's blow hole and saves its life.

Video Games

 * Brad Vickers of Wild ARMs 2  He proves himself more than capable of carrying the mantle once he regains his confidence, however.
 * In Chapter 3: The Paper Hero of Mega Man X Command Mission, X and Spider go to rescue some POWs inside a prison. There they meet up with a hero in green armor Steel Massimo Massimo begins the game pathetically weak, with average LE but no offensive or defensive strength to speak of...at LEVEL 1. And even at level 1, his LE is close to that of X at level 10.
 * Before the start of 'Medievil'', Dan boasts about being a great hero, and is remembered as one, but really isn't, by the end, however, he's truly become the hero he claimed to be.
 * Captain Qwark of the Ratchet and Clank series. Throughout the series he boasts about how amazing and heroic he is, but always fails to live up to it (he's even a villain in the first game). In the third game however, he fakes his death, but when Ratchet and Clank find him, Clank tells him that there's still a chance for him 'to be the hero he always wanted to be'. Cue his entrance during the final boss battle in which he helps you fight Dr Nefarious.
 * He's also helped out in the more recent Crack in Time, despite maintaining his pathetic qualities.