Allied to your point on applause, James, I've noticed that the level of joining in on choruses is often inversely proportional to how well the leader sings. It's as if the "audience" are over compensating for the singer's inadequacies and are trying to help them keep in tune/keep in time/go faster/go slower. Or just drown them out. Whereas a really magical singer will be more likely to be listened to: even if there's a chorus which you'd expect to be joined in on.