My agent in London told me, after Never Let Me Go, because I loved doing that so much, "If you're on a lucky streak and you're doing well, you should only take a part, if you can't bear the idea of anyone else doing it. " That's been the case since then, with Drive and Shame and the play (The Seagull), and the stuff that's going on, like Gatsby. I would have been devastated, if I hadn't gotten those jobs.
When there's no clear option, it's better to do nothing.