"It's got to be a challenge but at the same time you have to feel as though you can play them - it's really dangerous to want to be a part of something just because you think it's going to be great. I've been sent plenty of scripts where I've known that it's going to be a great film and a successful one, but I just couldn't convince myself that I was the right person for the part. So, I think you have to be careful with that."