The problem that I see with this is that for every time I can think of a movie having been improved by deviating from the book, there are often other people who disagree.
I think that, realistically, the only sensible goal for the movie maker to choose when adapting a book to a movie is to maintain the ‘feel’ of the story. The characters are going to be basically the same. But even there, there are times that a character will need to be edited - a book, for example, can take the time to indulge in making a character terribly ambiguous, while a movie often doesn’t have the time to do that. The bare bones of the plot will remain the same, but even there editing will come into play. Whole subplots by the ream will be removed from the story of the book to make a 90 to 120 minute story for the audience.
In some ways, I happen to think that the filming of Dune is a particularly good example of what can happen if the filmmaker is too ambitious in trying to translate all of the book to the screen. When I saw the theatrical release in the theatre, I, and a friend, who’d also been familiar with the book enjoyed it. We weren’t all that impressed, but we enjoyed it. Of course, we also understood a lot of things that weren’t properly explained on screen - and had to provide all sorts of crib notes to our other friends who hadn’t read the book. I’ve heard that the longer director’s cuts are far more watchable, but the original film was just too much for most audiences to follow.
Granted, Herbert’s world was a bit more complex and needed more exposition to educate the viewer than would be needed in something set in contemporary American culture, and that played a big part in the problems the filmmaker had.
I think that part of the problem is that movie adaptations have a real hurdle to overcome: a novel is usually too complex to translate to the screen directly as a single session. I believe that the proper equation should be a feature length movie is roughly going to be equal to the same story complexity of a short story or at most a novella. There just isn’t the same room for exploration of the world that there is in a novel.
Another example would be Dumas’ The Three Musketeers. The novel portrays, for example, Richelieu as a particularly ambiguous character, but not as the main antagonist for the characters. The main antagonist is instead The Countess. Richelieu actually ends up supporting them. Both movie versions of the story (If I recall correctly) reduced things to make Richelieu the sole villian. (He’s got minions, of course, but none with independant will or goals.) It’s not an unreasonable simplification, but it certainly changes the story considerably.
However, I believe that both films did achieve the intent of remaining true to the ‘feel’ of the story. Which is why that I’m not going to claim that they weren’t untrue to the book.
It’s a hard line to walk, and often the filmmaker trips. Of course, there are times, too, that the filmmaker just takes the title of the book, and then goes off in his or her own way for the actual story. (Not that I would point fingers, but Starship Troopers or I, Robot come to mind.)