Daniel Wallace, who wrote the book Big Fish, on movie adaptations:

The best adaptations are inspired by the source material, not dictated by it. The screenwriter doesn’t work for the author; he takes possession of the story, and owns it as much as the novelist does. If not, if there is even a hint of subservience, the adaptation and the movie it hopes to become will suffer, and sometimes die a grisly, unliterary death. And if you’ve never seen an unliterary death it’s a sad thing to watch.