From his Q & A about Westlake with Levi Stahl of the University of Chicago Press:
LTS: As you mentioned earlier, Westlake wrote screenplays—including an Oscar-winning adaptation of Jim Thompson's The Grifters. But he never (as far as I know?) wrote any screenplays based on his own material—even as he was never fully satisfied with any of the films that were made from Parker novels. Was there a reason for that?--Marshal Zeringue
BG: Several reasons. One, obviously, is that if you've written the novel then you've already told the story. Writing it in another form can be boring. It's much more interesting to adapt someone else's story for the screen—you haven't written it before. Another, probably less obvious, is that if a studio or producer buys your book, then it's their (or his) movie to ruin. If you write the screenplay, you're likely to get blamed if it's a bad movie based on your own novel. As Don said, "If I write a novel, I'm a god. If I write a screenplay, I'm a minor deity."
It may be true that Don was not fully satisfied with any of the Parker films, but he did like Point Blank a lot—we talked several times about director John Boorman's imaginative use of imagery and time, such as the scene in which Lee Marvin is shown waiting in a room, and then is shown waiting in the same room but this time it's unfurnished—like the character's mind. I don't think Don was crazy about the Alcatraz frame for the film's story—it struck him as pretentious—but he liked Marvin and he liked most of whatBoorman did with it. Also to some extent he liked The Outfit, partly because of its casting—director John Flynn cast Robert Duvall in the lead, and filled the 1973 movie with film noir actors from an earlier time, such as Robert Ryan, Marie Windsor, Jane Greer, Sheree North, Richard Jaekel, Tim Carey, and Elisha Cook Jr.
The rest of the Parker movies were routine except for Made in USA, and adaptation of The Jugger by Jean-Luc Godard that was incredibly bad—so bad that Don sued Godard in French court, won the lawsuit, and prevented the film from being mass-exhibited in the United States for many years. (You can get a copy now on DVD, but unless you're a masochist it ain't worth it.)
He never sold the Parker character, so the leading man in each of the movies has a different name. This was largely a commercial decision—if you give up the character, you may have given up all the books. (Joe Gores and I ran into that silliness when we tried to sell a Sam Spade screenplay.) But Don remarked more than once that...[read on]