The boy he tutors is changed from a high school teenager to a child. The fact that Sarah is in the movie far more than the book and campaigns for Stillson. The fact that Johnny's premonitions about Chuck are changed from a fire at a graduation party to a drowning at an ice hockey game. The fact that it's Sarah's baby that is used as a human shield by Stillson. I'm sure there's more, but I haven't seen the movie in many years.