It's important for a villain to be as threatening as possible, whether physically, mentally or emotionally ... however you want to do it. If you can combine all three, well that's the ultimate villain.
People come up to me and say, 'You are such a great BAD guy.' The fact is that the antagonist in a movie is usually the most fun to play. You can stretch the role and do so much with it. You can be boisterous. You can be insane. You can have a temper. You can do nothing but sit there and give a look and still be the bad guy. It's open to so much creativity. I try to change my physical appearance for every role.