Attention Deficit Disciple

So there's some sort of fun musical number going on and one character, usually a villain, is not joining in on the fun. They're either watching with a scowl, or have something much more important on their mind. The character looks over to his lackey or partner, only to see them dancing along! Often followed by a sharp glare and the lackey sheepishly stopping.

Manga

 * Zatch Bell: Folgore's first appearance. Kiyomaro is still analyzing him while he dances, and Gash couldn't help to join the dance.

Film

 * Occurs in Rocky Horror Picture Show when Rocky, as well as all the nameless Transylvanians and Columbia, start dancing along to Eddie singing Hot Patootie much to Frank's dismay/jealousy. Instead of stopping in embarrassment, Rocky gets thrown into the elevator by Frank and watches in horror as he kills Eddie with a pick axe.
 * The finale of the movie remake of Hairspray does this with Velma and Amber.
 * In Enchanted, Robert catches himself bobbing along with Giselle's Crowd Song before realizing what he's doing.
 * The Mask, with all the cops joining in. Subverted, too, when the lieutenant in charge tells his subordinate: "You go dancing out there and I'll shoot you myself."
 * In Anchorman, Brick begins joining in with a rival news team in laughing at his friends.
 * In How the Grinch Stole Christmas, the Grinch is displeased to find his faithful dog Max dancing along with the citizens of Whoville's song.

Live Action TV

 * TV's Frank does this numerous times on Mystery Science Theater 3000.

Western Animation

 * The SpongeBob SquarePants Movie involves this and a mosh pit.
 * In one episode of The Simpsons, Homer responds to Lisa's new vegetarianism with "You don't win friends with salad!", which Bart turns into a conga line. When they come back in-frame, Marge is part of the line; Lisa admonishes her, and Marge responds "Sorry, it was catchy". Something similar happens in the episode where Mr. Burns steals the family dog's puppies. He sings a song (to the tune of "Be My Guest") where he details how he plans on skinning them to make a tuxedo. Lisa is horrified, but Bart is humming along, and again, when Lisa calls him on it, he says "You've gotta admit, it was catchy."
 * Iago in Aladdin.
 * Max in Cats Don't Dance.
 * Occurs in The Great Mouse Detective during the mouse burlesque show, with Basil looking over to see Dawson (who's acting loopy after drinking a drugged beer) dancing with the showgirls.