Come on now. This was awesome casting. They needed someone to play a brainwashable psyco and in walked Tom. This movie helped shoot him to stardom, and he didn't even have to act. Somebody saw something in Tom before he was 20.
Tom Cruise is famous for his exaggerated walk, his aide even parodies it in Magnolia.
Classic small man's walk, and Cruise is the prototypical small man.