You can't generalize on how people will behave. Even basic survival instincts are frequently (and humorously!) subverted in Real Life; and Hidden Depths or a Mysterious Past mean characters have a lot of leeway to act Out of Character. However... there are many things people are still unlikely to do, especially when previous characterization has given them some measure of an attribute in contradiction to their new behaviour.
Below are a list of tropes relating the different ways characters break Willing Suspension of Disbelief by pulling a 180 and doing really weird, nonsensical and/or stupid things. Why does this happen? Two reasons: the author sucks, and drama requires them to.
- All Is Well That Ends Well: Something bad happens, but it all works out. Because of it all working out, the character who the bad thing happened to isn't annoyed.
- Artificial Atmospheric Actions: Background characters doing things in video games that are supposed to look realistic but instead look unrealistic.
- Chainmail Bikini: Chainmail that only covers up the places a bikini would cover up.
- Character Check: A character starts to act out-of-character but then does something which signifies they're back to normal.
- Character Derailment: A character suddenly and inexplicably changes personality.
- Conflict Ball: A character starts a conflict for weak or out-of-character reasons.
- Cybernetics Eat Your Soul: A person loses their morality when they become a cyborg.
- Distress Ball: A plot resolves around a character ending up in danger and needing rescuing, but the character gets in trouble in the first place by doing something dumb and/or out-of-character.
- Easily Forgiven: Someone forgives another person, even when what the person being forgiven did was very evil.
- Going for the Big Scoop: A reporter willingly goes right into danger to get a news story.
- Hero Ball: A hero who is shown not to be dumb or absent or uneducated but makes a bad or dumb-seeming decision anyway.
- Hilarity Ensues: A dangerous situation is Played for Laughs.
- Idiot Ball: A character does something stupid or foolish even though they're not dumb and they have the information necessary for them to know it's a bad idea.
- I'll Never Tell You What I'm Telling You!: A villain says that they would never reveal information, all the while revealing it (like "I'd never say that the answer is X")
- Little Professor Dialog: Children using big words.
- Looks Like She Is Enjoying It: Someone screams or moans in pain and another person thinks the screamer is having an orgasm.
- Otaku Surrogate: In a work with an Improbably Female Cast, a woman acts like a stereotypical fan of shows similar to the show she's in.
- School of No Studying: A work is set in a school but the characters are hardly ever seen studying.
- Smart Ball: A character suddenly appears much smarter or more knowledgeable than their usual selves.
- Stripperiffic: An outfit that looks like something a stripper would wear and is impractical.
- Undercover When Alone: An undercover character keeps acting even when they're by themselves.
- Villain Ball: A villain does something just to be evil which ends up being foolish and defeating them.
- Welcome Back, Traitor: Someone betrays another character but is welcomed back instantly.
- What an Idiot!: A YMMV page where viewers call a character an "idiot" for doing something perceived by them as foolish.