This trope is most common in musicals, but is not strictly relegated to only musical fare.
You have a happy-go-lucky or other highly charismatic character who is getting everyone around him/her all excited about whatever it is they're excited about.
Then there's the one character who isn't swept up. They're worried that everyone is all excited and not thinking it through. They are concerned that there's something shady under all the happy excitement. They're just grumpy and think all this excitement is too silly and twitterpated.
But as the song/scene goes on, the excitement spreads, and the enthusiasm grows. The lone holdout will find themselves going along and either catch themselves with embarrassment and stop — or will not catch themselves and end up going with the flow.
Compare with Not So Above It All
Anime and Manga
- Aversion: In Mahou Sensei Negima! Chisame when everyone is making a Hooray for [Grinding] until she starts thinking of them as friends (With the [Heroic BSOD] Chapter 176 pages 04-05.
- In Beauty and the Beast, Cogsworth is the only character who thinks that everyone else singing "Be Our Guest" and treating Belle with food behind the Beast's back is a bad idea and tries to stop them. He eventually joins in on the singing and dancing near the song's end.
- Enchanted: Giselle starts singing "That's How You Know" — Richard is embarrassed and annoyed. Then he's shocked when the local musician buskers join her. By the time Giselle has gotten most of Central Park involved, he's sarcastically playing along. By the final chorus, though, he's smiling and bobbing his head along with the tune, until he catches himself with a mortified "what the hell am I doing?!" expression.
- In Tangled, Flynn Rider watches the majority of "I've Got A Dream" sung by Rapunzel and the Snuggly Duckling thugs with a you've-got-to-be-kidding-me expression on his face. His eventual participation in the song isn't exactly voluntary — he's basically forced into participating at swordpoint after initially saying that he doesn't sing — but it is implied that he became swept up enough in the song's sentiment to respond later to a thug's exhortation to "follow your dream" with a genuinely enthusiastic "I will!" (the thug was actually addressing Rapunzel and thought that Flynn's dream sucked, but still).
- Kamina appears to have this power (as noted by the author) in DOUBLE K.
- The Shove in Unseen Academicals: a single moment where all humans are united in a single emotion (to the point of creating false memories), including the wizards unused to such emotional outbursts.
- In the 2013 West End musical Charlie and the Chocolate Factory, Mrs. Teavee — a Stepford Smiler Housewife who spends her life trying, mostly in vain, to control her Enfant Terrible son Mike — spends most of Act Two as the Only Sane Person among the Golden Ticket tour group. She's genuinely scared by both The Wonderland that is Wonka's Factory and the dreadful fates of the other three bratty kids. Finally her son zaps himself into Cyberspace via the Television Chocolate setup, and as the Oompa-Loompas begin The Villain Sucks Song "Vidiots" she notes "The little people are singing again. That's never a good sign." However, the anxious woman gets swept up in the electronica-influenced song, and she not only dances along but at the urging of Willy Wonka and the Oompa-Loompas sings as well in the final stretch, which predicts a potential Fate Worse Than Death for the boy if he can't be found. When the song is done, she's exhausted but exhilarated by the experience, and with regards to Mike — who's been found, but is now miniaturized — she's quite happy to have a son who won't be able to get into any more trouble.