After seeing Rothbart as a nightmarish figure in Nina's hallucinations for the whole film, it's quite a shocking laugh when the actor playing him in the show passes by Nina in full costume and gives a casual "Hey."
Seeing Nina's "homework" come to a jarring halt when she realises her sleeping mother is right there can be pretty funny.
Thomas' reaction when Nina bites him. Just the way he says "That fuckin' hurt."
Also, his reaction when the lights randomly going off when he's in mid-rant.
Nina's attempts to flirt at the bar consist of . . . offering to comp tickets to her performance. You can't HELP but laugh.