Near the end of his life, and long after his greatest novels were written, James M. Cain agreed to be interviewed by a student reporter who covered culture and the arts for his college newspaper. This young man began his time with Cain by bemoaning how Hollywood had changed books such as The Postman Always Rings Twice and Double Indemnity. Before he could properly get into his rant, the old man interrupted him by pointing to a shelf of books behind his desk. “The movies didn’t change them a bit, son,” he said. “They’re all right up there. Every word is the same as when I wrote them.”
I'll add that this is worth a read:
You're Not Entitled to a Faithful Adaptation