There is an awesome article online at BBCAmerica about David Mitchell's take on the screen adaptation of his book, Cloud Atlas.
Go to the BBC link to read it but the gist of it is Mitchell says 'if the movie was structured like the novel it would suck'.
He also acknowledged: "Each art form does bring certain limits to the table,” Mitchell said, and those making adaptations ignore them at their own peril."
Paul Hechinger, the BBC America reporter notes that Mitchell even came up with a list of five “habits of successful film adaptation” from an article in The Wall Street Journal called “Translating Cloud Atlas Into the Language of Film”
5 Habits of Successful Film AdaptationFirst, “bagginess” is great in a novel, but a movie has to “deliver plot more quickly.”
Second, details and atmosphere can be “suggested” in a novel; in a movie, “it’s either shown or it isn’t.”
Third, novels can have dozens of characters, but movies have to pare them down, or most viewers will be confused. He gives this habit the name, “Honey, I Shrunk the Cast.” Mitchell views the upper limit of major characters somewhere around eight.
Fourth, never underestimate the power of music. Novels may have figurative music, but the actual music of movies is transformative.
Fifth, books can leave lots of aspects open-ended. In movies, as a general rule, “all roads lead to closure.”
I don't know if I agree with number five. There are lots of open-ended films aren't there? Any favorites?