Basic Trope: A seemingly dumb or incompetent character is revealed to actually be very strong or skilled.
Straight: Bob is the more... Outlandish member of the heroes, but during the final (or otherwise important) battle, he shows to have superhuman strength.
Exaggerated: Bob is Too Dumb to Live, to the point where he can't even solve very simple problems. However, during the final battle, he starts flying, shooting fire from his hands and lasers from his eyes, and causes massive explosions by snapping his fingers.
Bob tries to avoid using his super strength because he doesn't want to hurt or kill innocent people.
Bob is a Heart Broken Badass and acts like a moron to entertain himself to prevent him from being depressed.
Inverted: Bob is very smart and respected, with people believing him to be a Bad Ass, but he is actually very weak and incompetent during actual battles.
Subverted: Right when it looks like Bob is about to turn his Bad Ass mode on, he trips and ends up doing absolutely nothing.
Double Subverted: He then gets back up and, a few seconds later, finally starts walloping on the Big Bad.
All the heroes are idiots (or otherwise quirky) but become super strong during battles.
Bob gains superpowers after his comedic Berserk Button is pressed and defeats the enemy who pressed it. He remembers nothing.
Zig Zagged: Bob turns into a Bad Ass while fighting the Big Bad, but isn't quite strong enough. Then Jacob, another seemingly stupid character, steps in and proves he is also a competent fighter, so he and Bob team up and defeat the villain.
Bob is just as smart and strong as everyone else.
Bob is stupid, and that's all there is to it.
Enforced: "The audience doesn't like how we're making this one character dumb. Let's make him suddenly whip out a superpower in the final battle!"
Lampshaded: "Dude, how'd you do that? I thought you were just an idiot!"
Invoked: "Everybody thinks I'm just an idiot. So I'll secretly train really hard, then when they least expect it, I'll prove I'm a lot stronger than they think!"
Exploited: Bob uses Obfuscating Stupidity so that the villain will overlook him and he can save his friends.
Defied: Bob tries to become super strong, but the villain kills him before he can.
Discussed: Bob reveals to one of his partners that he has a secret power that he's waiting to use, since it's very tiring and difficult to control.
Deconstructed: After learning thousands of deaths could have been avoided had Bob acted sooner, they immediately lose all trust in him...
Reconstructed: ...but it turns out Bob is Unskilled, but Strong, not even realizing he had that ability to begin with. They decide that what's past is past, and work on training Bob so that his ability can be better utilized. He still acts like an idiot during down time, but gets his act together when things get serious.
Played For Laughs: Bob is a hilarious idiot, then turns into a Bad Ass that is still stupid. He makes funny and irrelevant comments all while kicking the villain's ass, then ends up forgetting the whole thing and going right back to being an idiot.
Played For Drama: Bob is actually Obfuscating Stupidity because he doesn't want to seem strong to the Big Bad, or he'll get killed first. He wants to fight the villain himself, because the villain actually killed his father. He's been saving this strength for the final battle, ready to slay the villain for doing this. After doing so, Bob dies, having used up all of his power, but he's glad because he avenged his father's death.