One of the most controversial subjects in the fiction world for readers is the adaptation of a novel into a movie. Because Hollywood is notorious for pandering to a very simple dynamic and adjusting works to the same formula, many adaptations fall short or fall apart. And this has been, in particular, a theatre season for revisions rather than new visions: remakes, adaptations, and sequels.
Despite the pitfalls and angry fans, the disappointment of a beautiful and compelling idea done wrong, my favorite quote on this matter comes from Orson Scott Card. Card's had his own difficulties with Hollywood and luckily stuck to his guns. A group who wanted to film Ender's Game, for instance, had the grand plan of making him sixteen and giving him a love interest. But ultimately, Card has said, "The book's still there." No matter the quality of the adaptation, you can go back and the words, lines and events are still there unchanged.
More than that, there is the hope that even a bad movie will draw readers to the original book. I'm going to presume that Scifi's hack of "Earthsea" falls into that category ...
Someone else whose name, alas, I cannot recall, said that to be successful, a movie adaptation should have some surprises for the person who has read the book - it should, in some sense, stand on its own. I once took a screenwriting course and became acquainted with the requirements of the standard screenplay: strictures of acts, high points, and discoveries. It can't be an easy task to mold a novel to this form, and I'm sure there are movies that have failed because they have stuck too slavishly to the formula and ignored the altered tension points of the visual medium.
So the dangers here are multitude. How easy to depart too far from the original text; how easy to follow it so closely the result neither interests fans nor engages newcomers. I can only assume the way to make it work is to find the flavor, the underlying theme, and preserve the high points - and even that is a highly subjective process.
Anyhow, a few comments on specific movies:
MEMOIRS OF A GEISHA: I saw this movie before I read the book, so my perspective is a bit skewed, but I found this a very engaging, accessible movie - and found after I read that the movie had adhered quite nicely to the core of the book, removing some minor color events for simplicity and movie length and adapting others to create a more direct line of dramatic flow.
THE PRINCESS BRIDE: I put this one next as it's the other movie where I encountered the screen version first, as I'm sure is true for almost everyone. This is one of those rare cases where the movie surpasses the book. While there is some wonderful wit and sarcastic humor in the book, the character interplay is insular and truncated. Forced into dialogue thread by the constraints of film, the movie version is far more appealing.
THE PUPPET MASTERS: This movie went wrong in a number of ways, some of which were unavoidable. Heinlein's original novel (which I'm not much of a fan of to start with, admittedly) was set in an earth near-futureverse, and some of the story had to be axed to shift the setting to modern earth. Without that setting, they also had to make Mary a brilliant scientist - which isn't necessarily a bad thing, as the book was very much a product of its time in regard to women. The worst part, however, is when the screenwriters chose to take the most interesting parts of the Masters out and strip them down to the pieces that left them, basically, generic horror monsters. Only three or four scenes survived from the book into the movie, and the main denoument is bizarre and largely unsupported by the original. I mainly tolerated this movie because ... Donald Sutherland.
HANNIBAL: ... huh? Some of the cuts made in this movie helped the storyline - I wasn't sad to see Mason's crazy sister get no air time - but mostly, they just diluted the core themes to the point where the original ending just wasn't believable. Even though most of the individual scenes were rendered fairly faithfully, the overall threads didn't pull together into a movie that was worthy of the source ... and unlike Silence of the Lambs, it leapt over the gore line. I think there were plans to make a sequel to this to take the main characters to the actual ending of Hannibal, but they never materialized. One thing I can't fault: Julianne Moore made a fantastic Starling.
LORD OF THE RINGS TRILOGY: This is arguably the most famous adaptation, certainly in the fantasy genre, and despite quibbles such the artificial interjection of Arwen into the movies, I think these movies do at core what they are supposed to do: catch the spirit of the original while still providing elements that surprise and engage fans. Some unfortunate things happen that I believe are because of the medium ... for instance, Faramir's reaction to the ring is often pointed out to me as being counter the books. However, without subtext and narrative, it is difficult to show the ring's drawing power if an inordinate number of characters just brush it off. I also agree with those who say the battle scenes took up too much of the movie, and there I intend to offer no defense.