Believe me, there is a ton of stuff we shot on Superbad that was unusable, because people were just riffing and riffing. It's just part of the Judd Apatow method, and part of the technique is to also be able to rewrite the movie again in the editing room.
So the actual riffing came out of us just sitting there and doing it the way I think some people think we really did it, which is all spontaneously, and it really was.
I do lots of crowd work in my set, because I enjoy writing material through riffing and conversation.
In a weird way, riffing on genres is kind of a reaction to formula. When you watch so much of the programmers and the films that you just think you've seen before, it's kind of going back to the well in terms of trying to conjure up the spirit of what made you excited about films in the first place.