I've learned, having been on a lot of sets, the good news is that by definition you are surrounded by experts. They get fired if they're not - unlike in the theatre!
I think the thing about film is, as it gets proved by a lot of young filmmakers now, that the medium will just go on reinventing itself, and so you just hope to be a part of that and not a part of some kind of endless regurgitation or 'Here I am doing what you know I do' kind of thing.
You can act truthfully or you can lie. You can reveal things about yourself or you can hide. Therefore, the audience recognizes something about themselves or they don't -- You hope they don't leave the theatre thinking that was nice...now where's the cab?'