Just watched the movie; and yes it was awesome. Leon's change of expression bothered me at first. But when you think about it, he has to be emotionless, calm, and quiet. Noticed that he never engaged in dramatic or serious conversations with other characters and just observes. He can't get involved with them and risk leaking information. His face is so solid that it is like he went through those hellish interrogation training when becoming a special agent for the president.
In RE4, he has much of an ego. Probably because he has to act differently during other missions (different personality). And plus, he was alone and the only sane-people he talks to was Hunnigan and Ashley. It was also his first assignment. He was fresh. XD