Often deviating from the original material changes the story so much it isn't the same story anymore. But to be fair, a film and a novel are two very different mediums and often one does not translate to the other. Often one can not cover the range of character development or emotion in a film that a writer can in a novel. Also book production is not the commercial venture that the production of a film is, so pandering to the audience, as far as what they want to see and find acceptable, is far more necessary in a multimillion dollar film than in the publishing of a novel. All that being said, one of my favorite books (Hannibal, by Thomas Harris) was well handled in the film version up to the ending. Then the entire story was made trite and formulaic because Ridley Scott and the studio were convinced that the audience wouldn't accept the author's dark and unusual ending.