Likely the biggest change from the book to the movie is in the outcome of the final fight and the participants in it. Jerry fights Archie, instead of Emile, and instead of losing the fight in rather spectacular fashion, he wins it and feels bad afterwards.
This is a rather large deviation from the ending of the book and suggests some very different things. Instead of Archie remaining the leader of the Vigils and them remaining in power, there is some breakup and loss of their power in the movie version. Some argue that this completely perverts the author's intent in terms of the story. Perhaps it was karma that then made the film rather unsuccessful!