Some classic Archetypal Characters or characters with a trope associated with them who might be the cause of this:
- The Actual Pacifist starts a fight.
- An Affably Evil villain drops all signs of niceness entirely, particularly during a Villainous Breakdown.
- The Affection-Hating Kid wants a hug or doesn't mind seeing people hug.
- The Alcoholic/Hard-Drinking Party Girl puts down their drink for the time being (or dumps it out and swears to never drink again).
- The All-Loving Hero wants to hurt/kill someone.
- The Aloof Big Brother is suddenly nice to their younger sibling(s).
- The Alpha Bitch doesn't antagonize anyone, including their usual targets.
- The Animal Lover acts mean towards an animal, or even wishes death on one.
- The Annoying Younger Sibling isn't acting obnoxious or rude to their older sibling(s).
- The Anti-Hero decides not to do their common anti-heroic trait.
- The Apathetic Teacher becomes more engaged with their student(s).
- The Apologetic Attacker offers no form of apology and/or regret from hurting someone.
- An Arrogant Kung-Fu Guy acts humble or meets an opponent they state is better than them.
- Someone with Attention Deficit... Ooh, Shiny! stays focused on the task at hand.
- An Attention Whore doesn't want to be the center of attention or doesn't notice when they are.
- An Ax-Crazy person is both calm and doesn't want to get violent.
- The Bad Boss doesn't harm or threaten to harm their underlings. Double points if they do something nice for them.
- The Benevolent Boss is unusually mean to their employees.
- Anyone with a notable Berserk Button don't react with the usual anger when the button is pressed.
- The Berserker/Hot-Blooded guy calms down, or goes into Tranquil Fury.
- The Best Friend antagonizes their best friend or acts like they don't know them.
- The Big Brother Bully doesn't antagonize their younger sibling(s).
- Someone who has a Big Brother Instinct doesn't step in to defend and/or protect their younger sibling(s).
- The Big Eater states that they're not hungry, double points if they turned down their Trademark Favorite Food.
- The Bitch in Sheep's Clothing is genuine about their kindness with no ulterior motives.
- The Blood Knight suggests a peaceful solution or is repelled by violence.
- Someone with Blue-and-Orange Morality conforms to normal standards of morality.
- The Bore does something interesting.
- The Bratty Half-Pint is genuinely polite and well-mannered.
- The Bully doesn't target and/or is nice to their victim(s).
- When there's A Boy and His X, the boy or the X starts acting strangely towards their respective buddy.
- The Casanova, the Lovable Sex Maniac, the Good Bad Girl, the person who Really Gets Around, or the Chivalrous Pervert shows disinterest in sex/romance.
- Somebody with a catchphrase doesn't say it at a time when they ordinarily would or says something else instead of their catchphrase (sometimes even seeming like they're trying to say it but they can't) at a time when they'd usually say their catchphrase, or says their catchphrase in a different tone of voice from usual. Alternatively, someone (who may or may not have one themselves) says somebody else's catchphrase.
- The Chessmaster makes use of the Indy Ploy.
- The Clingy Jealous Girl, Crazy Jealous Guy or Yandere doesn't act out when someone makes moves on their love interest.
- The Clock King or the Creature of Habit deviates from their routines purposely.
- The Cloud Cuckoolander begins to talk and/or act logically.
- A Comically Serious person tells jokes or laughs at someone else's.
- The Confidant doesn't want to give advice or gives bad advice.
- The Combat Sadomasochist tries to defeat their opponent quickly and takes no joy in it.
- The Consummate Liar is unable to hide the truth from someone.
- The Control Freak doesn't want to be in control or wants to be chaotic.
- A Cool Big Bro starts acting more cold or even mean to a younger sibling.
- The Cool Mask wearer suddenly removes it; one normally in makeup being without also fits this.
- The Cool Teacher lacks interest in teaching their student(s).
- The Cuddle Bug or person with No Sense of Personal Space respects someone's personal space.
- The Cynic starts to be optimistic.
- The Daddy's Girl is distant with her father. Double points if she is either angry or disrespectful with him.
- The Deadpan Snarker is at a loss for words.
- The Determinator gives up.
- The Dirty Cop does their job professionally.
- A Dirty Coward/Lovable Coward stands up to fight.
- A practitioner of Dissonant Serenity loses their cool.
- The Ditz does something distinctly intelligent and/or sensible.
- A character who Doesn't Like Guns uses one.
- Someone with a Drink-Based Characterization chooses not to have their favorite drink.
- The Eeyore becomes genuinely happy and behaves accordingly.
- One whose eyes are always shut opens their eyes, even if only for a second.
- The Extreme Doormat stands up for themselves.
- The Fearless Fool becomes scared or admits something is too dangerous.
- The Fettered or The Unfettered compromises on their ideal or goal.
- The Friend to All Children starts being mean to children or believes a child is too dangerous to keep around.
- Creatures stop liking, or even become scared of, someone who's usually a Friend to All Living Things.
- The Fundamentalist thinks someone on their own side is too extreme. Bonus if it's Big Bad and/or The Heartless one being disgusted.
- The Gadfly doesn't make fun of anyone but is completely serious.
- The Genki Girl becomes quiet and/or reserved.
- The germophobe goes somewhere or touches something filthy.
- The Glory Hound or Glory Seeker lets someone else take the credit.
- The Gosh Dang It to Heck! person swears for real.
- The Graceful Loser takes a loss poorly.
- The one with a Guilt Complex refuses to take responsibility for their actions or accepts that they are not at fault.
- The character with a Hair-Trigger Temper waves off a slight. Double points if they didn't react to, or even laughed at, their biggest Berserk Button.
- The character who Hates Being Touched allows someone to touch them (almost always in the form of a hug).
- The Heavy Sleeper is awakened by something that wouldn't normally wake them up.
- The Hero-Worshipper turns against their hero.
- The Indy Ploy practitioner actually takes time to plan things out.
- The Ineffectual Loner asks for assistance.
- The Insufferable Genius admits they don't know/are not smart enough to figure out something.
- The It's All About Me or Morally Myopic person shows concern for others, however grudgingly; the former tries to avoid the spotlight.
- A person who normally engages in Jack Bauer Interrogation Technique does not use torture on a person.
- The Jerkass becomes very kind.
- The Jerk Jock commits a genuine act of kindness. Double points if they do this towards their usual bully target.
- A Keet becomes subdued.
- The Knight Templar is horrified/ashamed by their actions.
- The Knight Templar Big Brother refuses to harm someone who hurt their sibling(s).
- The Knight Templar Parent doesn't harm someone who hurt their child(ren).
- A Large Ham tones it down.
- A Lazy Bum or Sleepyhead becomes active.
- The Leeroy Jenkins becomes extremely cautious.
- Somebody with a Limited Wardrobe wears different clothes for no reason.
- The Lovable Jock acts mean towards those they are usually nice with.
- The Lovable Traitor doesn't betray their allies.
- A Manchild acts mature.
- The Master of the Mixed Message is completely honest about their feelings, with no room for doubt.
- The Mean Boss does and/or says something nice to their employees.
- The Mellow Fellow becomes stressed and/or angry.
- The Momma's Boy is distant, angry, or disrespectful with his mother.
- The Motor Mouth shuts up.
- The Narcissist is either humbler than usual, shows empathy to someone else, or doesn't care for the attention he's given.
- Someone with Nerves of Steel or Tranquil Fury loses their cool or even goes into Unstoppable Rage.
- The Neat Freak doesn't clean up or doesn't care that something is dirty.
- The Nervous Wreck turns into a daredevil.
- Someone who is Never Bareheaded is seen without their trademark hat.
- The Never My Fault person admits It's All My Fault.
- The Nice Guy takes a level in Jerkass.
- The Nicknamer calls another character by their real name.
- The person with No Indoor Voice whispers.
- The Non-Action Guy gets into a fight. Double points if they're the one who started it.
- The Not-So-Omniscient Council of Bickering manages to act quickly and decisively. Bonus points if their decision is unanimous.
- The Omnicidal Maniac tells someone that You Will Be Spared.
- Children stop hanging around someone who's normally One of the Kids.
- The Only Sane Man acts wacky.
- The Boyfriend-Blocking Dad doesn't react when their child states they are dating someone.
- The Perfect Pacifist People rise up in arms.
- The Perpetual Frowner stops frowning and /or starts smiling...
- The Perpetual Smiler stops smiling and/or starts frowning.
- The Picky Eater eats a food they hate the most, and starts liking it.
- The Pig-Pen cleans up.
- The Plucky Comic Relief becomes dead serious.
- The Politically Incorrect Hero or Villain doesn't say anything bigoted about whichever group they are prejudice against. Take it a step further if they willingly partner with someone from said group with no complaint.
- The Pollyanna admits to despair.
- A character who Prefers Going Barefoot begins wearing shoes all the time.
- The Professional Butt-Kisser doesn't take the opportunity to get in their superior's good graces. Double points if they either act more disrespectful to said superiors, disobey an order, or don't even notice when said superior praises them.
- The Proper Lady speaks or acts improperly.
- The Proud Warrior Race Guy says a plan of action is too dangerous.
- The Pushover Parents discipline their child(ren).
- The Quiet One begins to talk more frequently.
- The Reasonable Authority Figure starts acting obstructive to their team.
- The Reliable One states that they can't help someone in need.
- The Reluctant Fanservice Girl lets loose or is unbothered by her nudity.
- Someone who dishes out Rhymes on a Dime stops rhyming.
- The Scrooge suddenly gives something away.
- Someone who speaks in Sesquipedalian Loquaciousness doesn't use big words or can't pronounce a big word.
- The Shameless Fanservice Girl dresses conservatively, and not for health reasons, or has a Naked Freak-Out when stripped.
- The Shipper on Deck is uninterested in how their favorite couple(s) are having a romantic moment together.
- The Shrinking Violet/Prone to Tears character becomes confident.
- Sir Swears-a-Lot doesn't drop even a single, mild swear.
- The Slacker is proactive.
- The Sleepless falls asleep, acts sleepy, or claims to be tired.
- The Smug Snake loses all of his/her haughtiness and becomes ashamed of him/herself.
- The Social Darwinist forgoes their philosophy.
- The Sociopathic Hero expresses genuine concern to others, self-shame, or compassion.
- The Sore Loser takes a loss surprisingly well.
- The Stoic becomes Not So Stoic.
- The Straw Character demonstrates actual tact, respect for their opponent, and logic.
- The Sugar-and-Ice Personality or Tsundere is upfront about their affection for the love interest.
- The Supreme Chef makes a food that nobody likes.
- The Teetotaler drinks alcohol, double points if they claim they needed it.
- The Terse Talker speaks in complete sentences.
- The Third-Person Person uses a personal pronoun/refers to themselves in the first person.
- One of Those Two Guys is seen without the other.
- One who practices Thou Shalt Not Kill takes a life.
- The Token Evil Teammate performs or suggests a more heroic solution for a problem.
- The Token Good Teammate performs or suggests a more evil or morally dubious solution for a problem.
- Someone with a Trademark Favorite Food refuses their favorite snack and/or meal.
- The Trickster Mentor clearly and directly tells their protege what they want.
- The Troll doesn't take the chance to drag someone down.
- The True Companions don't get along.
- One who normally likes to Turn the Other Cheek rejects someone's apology, double points if they said to their offender on that what they did is unforgivable.
- The Unapologetic gives out a sincere, heartfelt apology.
- The Understanding Boyfriend isn't supportive to their partner.
- The character with Undying Loyalty pays no attention to (or turns against) their master.
- The Ungrateful Bastard shows genuine gratitude for once.
- Someone with an Unlimited Wardrobe starts wearing only one pair of clothing.
- The one with a Verbal Tic speaks normally without dropping a single tic.
- The Violently Protective Girlfriend doesn't attack the person who hurt their lover. Double point, if the former is fighting against their latter.
- The Voiceless or The Silent Bob talks.
- The Walking Shirtless Scene puts on a shirt.
- The "Well Done, Son" Guy doesn't care that their action can lose their parental figure's respect.
- The character who bemoans Why Did It Have to Be Snakes? doesn't react to one, even when it's right in front of their face, or even attacks it.
- The You No Take Candle speaker talks with normal syntax.
- The Zany Scheme maker comes up with a sound and logical plan.