Who does Ben Barnes Play in Stardust (The movie)?

Apparently there's this movie called stardust, I remember that I read a book by the same title a little while ago, and Ben Barnes (and actor I really like after having seen him in Narnia (prince Caspian) is in it. I think the movie is based off of the book, and I may want to see it to see him in another movie. If anyone has seen the movie, can you tell me which character he was? I mean, I know the name, but I can't remember who that was in the book. Was he the main character, or what did his charcter do? If anyone knows, I would really appreciate some insight. Thanks!