Promise Me You Won't X

Homer: All right, Marge, I'll tell you, but first you have to promise you will not get mad.

Marge: I promise I will get mad, because I always do when you make me promise I won't.

—The Simpsons, "Lisa The Greek"

Two characters have a conversation like this:

Character A: I need some advice from you.
Character B: Shoot.
Character A: Promise me you won't laugh, okay?
Character B: Okay then.
Character A: How do I get rid of this rash in the shape of lovehearts on my left buttock?
Character B: Bwahahahaaha!
Character A: You promised you wouldn't laugh!
Character B: Sorry dude, I didn't mean to.

Sometimes Character B is Character A's friend; they may try to unsuccessfully stifle the laugh. If they're The Rival (or The Jerkass) then they'll probably just be in peals of laughter. "Promise me you won't laugh" isn't the only scenario, though; sometimes it's substituted with "you won't scream", "you won't panic", "you won't get mad" or something similar. Almost every time the person goes back on the promise. Beginning to be subverted more and more often in the style of The Simpsons quote at the top of the page, or by someone saying "I'm not promising anything till I hear what you say."

Another variation on the trope is a conversation that goes something like this:

Character A: What happened?
Character B: Trust me, you don't want to know.
Character A: No really, tell me.
Character B: *insert gross/inappropriate/silly/ridiculous story here*
Character A: ... you were right. I didn't want to know.

Examples of Promise Me You Won't X include:

Anime and Manga

Kanon, That's one way to reunite with your Not Quite Dead girlfriend...
In an episode of Full Metal Panic? Fumoffu, Sousuke forgot to bring Chidori's notebook to class, and he asked her to promise she would not panic no matter how bad the situation is. She kept that promise - she went into an Unstoppable Rage instead.

Film

Casper, (1995 movie) has Casper muffle Kat and explain his position as a friendly ghost, and offered to un-muffle her if she promised not scream. Of course as soon as she could speak again, she screamed.
- Averted the second time he does it.
The Sixth Sense: Cole and his mother are having a conversation about a pendant of her mother's. Cole's mother is sure Cole's taking the pendant. She tells him, "I promise I won't get mad," and she asks him if he took the pendant. He tells her no—as he tells her every time. And naturally, she gets angry with him.
Subverted in Waking Ned Devine: Finn asks Maggie to read some of her poetry (actually, greeting card texts) to him. She makes him promise not to laugh... and then breaks into laughter after her own first word.
Robin Hood: Men in Tights: Maid Marian pleads with Robin not to enter the archery contest that she knows is a trap. Of course, the promise of facing an undefeated archer is too tempting for Robin Hood.

Maid Marian: Robin, promise you won't go.
Robin Hood: All right, I promise you won't go.
Maid Marian: Thank you.
Ahchoo: But wait a minute, Robin, didn't you just...
Robin Hood: Cool it...
Ahchoo: Chilled.

Near the climax of M. Night Shyamalan's The Village, right before Edward Walker shows his daughter Ivy the monster costumes they keep in the shed: "Do your very best not to scream."
In Pulp Fiction, Mia Wallace calls Vincent out on asking her not to be offended by a question, saying that if she promises not to be offended and then is offended, he'll have forced her to break a promise.

Literature

In the Discworld book Going Postal, Moist von Lipwig reveals his Boxed Crook backstory to Adora Belle Dearheart, including the fact that he was the crook whose forgeries got her fired from her previous job at a bank. He first makes Adora promise not to make any sudden movements, because "I think that in a few seconds you'll try to kill me." Played with in that Adora does hold to this promise, with some reluctance.

Live-Action TV

Subverted in The IT Crowd. After a masseur kissed Roy on the arse, he was about to tell Moss but first made him promise not to laugh. Afterwards, Moss is disgusted that Roy would think he'd find it funny.

Radio

The Goon Show: In the episode "Shangri-La Again", Major Bloodnok is supposed to have obtained an aeroplane to fly the Embassy staff to safety:

Bloodnok: The plane...the plane. Ooo! Ooo heavens!
Seagoon: What's up?
Bloodnok: Look - if I tell, promise you won't blow up?
Seagoon: I promise.
Bloodnok: I forgot to order it.
GRAMS: Explosion.
Bloodnok: You promised!

Theater

Big The Musical has an extended sequence in the song "This Isn't Me" where Josh, suddenly an adult, asks his mom not to turn around yet, and not to scream when she does. She instead asks why she would scream, and of course does just that when she sees him.

Video Games

Fire Emblem has a support conversation between Eliwood and Hector, where Hector's thinking of a dream he had and Eliwood asks him to share it. His response before doing so: "Laugh and I'll kill you."

Web Comics

The Adventures of Dr. McNinja combines it with Implausible Deniability [1]^{[dead link]}.

Dr. McNinja: You can talk?
Motorcycle: Promise you won't be upset?
Dr. McNinja: Yes.
Motorcycle: Yes.

Western Animation

Braceface, "Leap of Faith" (last episode), "promise me you won't panic" (or "freak out", or something similar) when Alden tells Brock he's fallen back in love with Sharon.
The Simpsons, "Lisa The Greek", subverted (see quote).
Transformers Animated, Return of the Headmaster:

Sentinel: I need you to come to my position. And promise me one thing - that you won't laugh!
Optimus: [sees Sentinel - as a head, victim of the Headmaster, and burst into hysterical laughter]
Sentinel: You promised you wouldn't laugh!