With The Hobbit: An Unexpected Journey due out in theaters soon (er or later), a lot of fans are debating the merits and demerits of Peter Jackson's film adaptations of Tolkien's works - a debate with which I am intimately familiar, having argued the demerits frequently and fervently in my younger, more impassioned days. I'm not going to take up that argument again (I'm getting too old for this sort of thing), but I would like to reply to one of the most common rebuttals to arguments against the films.
"You can't make a film exactly the same as a book - that's why they call it an adaptation. Things that work in books don't work in movies - like you can't show the characters' internal monologues on film."
I'll agree with your premise, but disagree that the license of adaptation can excuse all of the choices made in the production of the films. What a truly good adaptation does is embodies the spirit of its source in a new medium - it shows a deep knowledge of and respect for its source, and gives its viewers the same feeling upon watching it that the book gives its readers upon reading. With that ideal in mind, I give you an annotated list of my favorite adaptations, which may help to elucidate my expectations.
Director Robert Rodriguez refers to his film as a "translation" rather than an adaptation, so it may not be quite fair to admire it as a pinnacle of the art of adaptation, but the translation is really quite impressive. Impressive not because the graphic novels were used as the film's storyboards, or because nearly every line of dialogue came from the novels. From the casting to the coloring to the set design, every step of production (which often involved groundbreaking production techniques) was calculated to be the graphic novel on film. Some would say it didn't make a very good film, but I do not think that its failures were due to the failure of the adaptation. I think that watching the film is every bit as good as reading the graphic novel, and if you (like me) have trouble getting your eyes to follow the action from frame to frame on the page, watching the movie will give you fundamentally the same experience.
The Silence of the Lambs
The book was a surprisingly literate crime thriller that developed who should have been a throw-away creepy villain (for that's about all he was in Harris's previous novel, Red Dragon) into a truly chilling, memorable character. The film, through skillful directing and impeccable casting, turned it into an unforgettably tense psychological thriller and elevated Hannibal Lecter to the very top of his profession. The increased effectiveness of the film did not come from changing anything significant about the plot or even the characters; I think it had more to do with the ability of film, at its best, to draw out more tension than literature. I estimate the film and the book as perfectly level in terms of artistry - no masterpiece, either of them - but the film strikes me as slightly more effective.
No Country for Old Men
This book was incredible. The film was every bit as good. I do not think it's a coincidence that the film recreates the book almost scene-for-scene (and occasionally line-for-line), but that fidelity is not its most impressive fear. Somehow the Coen brothers manage to translate McCarthy's iconic literary style into a visual style that works every bit as well. Most of the book is written in a down-to-earth, simple vocabulary, but the lexicon McCarthy uses to describe the landscapes sent me to a dictionary several times. Much of the film was filled with wide, sweeping vistas saturated with color and texture. In the book, the characters boots garner frequent mention as a sort of representation. . . well, a great many things. In the film, likewise, we have many shots centering on boots, and an entire scene about Llewellyn buying boots. Even the narration from Tommy Lee Jones matches the italicized chapter-openings, if not word-for-word (though, if I recall correctly, it sometimes does) then at least in tone. Once again, I would rank the artistry of these two works as perfectly equal. If I like the film more, it is only because I've seen so few films as good, when I've read many books as good. If I like the book more, it is only because I generally prefer reading to watching movies.
And those are all the adaptations that I've really liked. Adapting a good book can be done faithfully, though faithfulness has more to do with respect for the source material than with actual fidelity. Speaking of respect for source material, one last thought about The Hobbit films: If they really were made out of love and respect for the book, wouldn't they be subtitled "There and Back Again" and "A Hobbit's Holiday?" To invent a new subtitle implies that you really split this into two (when you managed to fit The Return of the King into one film) to make more money at the box office.