I found an explanation under an other video of the same scene. Molly Grimes wrote this: Molly Grimes vor 2 Jahren: The question was if they were to perform on Broadway hat show would they want to do and Misha said he'd want to revive Spiderman the Musical and this girl said she's seen it twice and Misha was like "why would you go see it twice??" and was laughing and apparently she's never even seen the regular Spiderman and she asked if Misha was making fun of her and he was like "yeah I am you're very astute" and Mark though Misha called her "a stout" which is apparently a thing from the Wind in the Willows and they were laughing at her being this stout thing and the girl said she was in the Wind in the Willows musical in middle school which led to them laughing even harder and she said she played "the friendly fox" which was originally supposed to be evil but she wasn't convincing enough as evil so they had to change it to a friendly fox at which point Misha started crying he was laughing so hard and that was when this video caught up with the laughing. (Sorry this is so long but it was a long story like they continued on that conversation for a while and Misha couldn't stop laughing haha)