When actors do plays, their union is an honorable and wonderful organization called Actors Equity that prevents a lot of mistreatment and non-payment. It also allows non-payment in small, unlikely-to-be-profitable situations. There is talk of changing that and I think it would be a very bad idea.
I would write a post explaining why but my buddy Ken Levine already has and I can't improve on what he said. You're probably already reading Ken's fine blog but if you aren't, read that post and then read everything else he's written.