In the LA Times, curmudgeon Richard Schickel discusses movie adaptations of popular books:
It's a very lucky novelist who creates a sacred text, something the movie folks dare not screw around with. Leo Tolstoy was not at first fortunate in this regard; the first screen version of "Anna Karenina" permitted her a happy ending. Neither was Herman Melville; there's an adaptation of "Moby-Dick" in which Ahab finds romantic fulfillment instead of a tragic fate in the final reel. Joseph Campbell, the great student of myth, once defined movies as "the genial imaging of enormous ideas," and these adaptations proved too genial — better make that too risible — for the literati of their day. Even Dashiell Hammett had to endure two ludicrous adaptations of "The Maltese Falcon" before John Huston finally got it right in 1941.
This suggests that when it comes to adaptation, neither literature's high end nor its low end is likely to gather about it an impassioned crowd whose demand for faithfulness weighs heavily with the studios. There are exceptions, of course. Back in the '30s, there were delightful movie versions of such beloved classics as "Little Women" and "David Copperfield," not complete in every detail but engagingly true to the spirit of those novels. And from the low end of the literary spectrum there was, in 1944, Billy Wilder's unimprovable take on "Double Indemnity," which its author, James M. Cain, thought (correctly) was better than his hastily written novel.