I don't really find things funny unless they're deeply tragic at the same time. I think if you're funny just for the sake of being funny, it's just frivolous nonsense. To me, all the best comic plays have been written about really serious and rather bleak things.
In many ways, theatre is more rewarding for a writer. I used to think it was like painting a wall - that when the play is finished, it's done - but now I realise it's more like gardening; you plant the thing, then you have to constantly tend it. You're part of a thing that's living.