Often films reflect the proclivities of the director, writers, and/or the producer.

Quickly comes to mind Elia Kazan's filmic version of the Steinbeck novel "East of Eden". Kazan based his film on the third section of the novel. He presented the father as a bitter, bible-thumping patriarch, about 180 degrees from Steinbeck's original character, in order to emphasize a disfunctional father/son relationship. It made a hell of a good film, but one that was not in accord with the novel. Steinbeck's book was better because of the generational sweep of the story; Kazan's was better as a character study, loosely based on the novel.