I approach every part I'm asked to do and decide to do from exactly the same angle: who is this person, what does he want, how does he attempt to get it, and what happens to him when he doesn't get it, or if he does?
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 have every sympathy for writers. It's a mystery to me what they do. I can edit. I can cross out and say, 'I'm not saying that' or, 'How about we move this to here? Wouldn't that make that bit of the story better?' But where any of it comes from is beyond me. I will never write a play or a novel.