Have you ever watched a supposed comedy film that started out really funny, but trailed off towards the end? Or towards the second half, in some cases? Of course you have - unless you don't watch comedy films at all or you have few enough brain cells that you'd be amused by people getting kicked in the groin. I always sort of assumed that this was because it was impossible to write enough comedy to fill, say, ninety minutes.
Then I started watching more stand-up gigs, both live and on TV/DVD. A lot of these shows are the same length, or longer, than the average feature-length comedy film. And yet they're able to keep the humour going throughout. I saw Russell Kane at the Hexagon a few weeks ago and could hardly breathe for laughing. Seriously, I had a weird pain in my side all the next day. He never ran out of material, I never got bored and needed a story (my other theory as to why comedy films stop being funny - they believe the audience needs an actually solid story.) (FYI, you can have humour and a story. They're not mutually exclusive.)
So I got to thinking, maybe being a part of the joke is important. A comedian is addressing the audience, characters in a film are pretending they don't exist. Seeing a comedian perform live is a strange thing, you laugh at things you'd never have normally laughed at because of the atmosphere. You think things are funny that normally you wouldn't. It's the same with TV shows - I can't watch comedies alone because they're just not as funny. Laughter is a communal thing. And I think maybe that's why comedy films don't always work - because the audience isn't really a part of the joke.