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.
- 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" — Robert 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).
- 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.
- On How I Met Your Mother Barney often gets really excited about some new thing, and will spend a lot of energy trying to get his friends to be just as psyched about it. This only works about half the time, though.
Everybody: "I said a-bang. Bang. Bangity bang. A-bang bang bangity BANG"
- A heartbreaking version of this trope plays out after Robin and Barney break up. Its been weeks/months since they split, and now Marshal, trying to encourage Robin to go out on a date with a great new guy, points out that Barney has moved on so she should too. He gets really enthusiastic explaining just how much sex Barney has been having, and starts singing a song about it. Ted walks over and joins in, then Barney himself, and eventually even Lily gets in on it. Later, when Lily is explaining to Marshall (and the other guys) that Robin has actually still been having a hard time with the break-up, they question why Lily joined in on their unintentionally hurtful song if she knew the whole time. She defends herself by pointing out it really was a very catchy song. Then they all start singing it again for a moment.
- During Season 17 of The Amazing Race, the teams had to do a Roadblock where one racer had to find a piece of fake food in a buffet full of real food. The task was set in a real restaurant in Hong Kong, with real local customers being entertained by singers on a stage. While her teammate Claire struggled through the task, Brook jumped up on the stage, started dancing, and stole attention from the singers. She got the restaurant patrons to chant Claire's name, and, when Claire finished the task, had the entire restaurant in an uproar.
- Season 21 had this happen again for another eating task, also set in China. The restaurant's patrons seemed confused at first to have all these Americans with camera crews eating hasma without using their hands, but twins Natalie & Nadiya pumped up the crowd with their Bollywood style dance, to the point that the crowd continued cheering for every team that came afterwards.
- 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.
- Played With in Avatar: The Last Airbender: "The Fortuneteller". Fun-loving Aang enjoys Aunt Wu's fortunetelling and hopes she'll predict he'll end up with his crush. The usually level-headed Katara is also swept up in the Aunt Wu fortunetelling mania. Which drives rationality-and-science-loving Sokka up a wall — he keeps trying to disprove it all as bunk.
- And in The Legend of Korra: Tenzin disdains pro-bending as a mockery of the noble history of bending. Until he sees that it works to teach airbending basics to Korra where he has been unsuccessful, and further, that it helps her team win the match. At which point he is swept up in the excitement of the jubilant crowd and cheers himself! Before collecting himself...and then going on to become an actual fan of the sport!
- My Little Pony: Friendship Is Magic: "The Super Speedy Cider Squeezy 6000": The Flim-Flam brothers' song, "Opportunity" causes everypony in Ponyville to get excited and start chanting "Cider! Cider! Cider!". Early in the song, Twilight and Spike stand out as watching this whole thing play out, wearing dubious expressions. By the second verse, they are still looking concerned. But the next time the camera returns to them, they're wearing big smiles and chanting "Cider!" along with everypony else.
- The Simpsons
Lisa: Mom!Marge: I don't mean to take sides, I just got caught up in the rhythm.
- It is subverted in "Monorail". Marge remains worried and uncertain and gets mocked for it.
- "Lisa the Vegetarian": Homer & Bart are mocking Lisa's newly found vegetarianism on the eve of Homer's BBBQ. Bart starts a chant "You don't win friends with salad!" Homer joins in creating a conga line. Then Marge joins.
- A fairly common occurrence in Phineas and Ferb:
- In "Tip of the Day", the eponymous characters sing a song to raise awareness for aglets. Candace is not impressed as usual at first, but as the song goes on she gets increasingly enthusiastic.
- In "Road Trip", Candace tries to bust the boys for starting a truck stop on top of the family's rented RV. She takes up the waitressing grudgingly. By the time they've gone through the song, she is looking at the order turnabout and herself like "what am I DOING?!"