My opinion: ask the director, but not in an e-mail. If you want to have a serious discussion with him, you need to see him face to face. Messages back and forth just don't cut it. And please watch how you word your inquiry. Don't ask, "How did Scary Boy get the part and not me?" That suggests that you don't trust his judgment, so why should explain anything to you? Instead, try, "What do I need to work on in the future to ensure that I get roles that I audition for?" That way you will get some insight into what he's looking for without attacking the actor he chose and without questioning his casting abilities. When dealing with temperamental musical theatre types, tact is everything.