The difference: They are disparate forms of communication. The most “honest and straight forward” interpretation of a book would be a a reading of its dialog by actors. With this most idiotic adaption of prose, the film would still convey an entirely different message, because film is primarily a visual medium. On top of this it would be unpleasantly long, as well as unsatisfying, because most literature details internal action more then it does speech.
So our hypothetical film is a real piece of shit that is still radically different then the original because film is a visual discourse, and most of the information is conveyed by the picture. There is no direct translation of text into film,

Here is where I think I might lose a few readers: This hardly precludes the possibility of a film being genuinenly ‘honest’ to the text. It simply means that do so there has to be expansion, compression, reconceptualization and interpretation.

Name me a film that is more honest to its text than Apocalypse Now, regardless of the fact that Heart of Darkness was written 64 years before the Vietnam war. Scorcese’s realization of the thematic ideas of Conrad are stunning, but from the standpoint of plot the film is hardly honest.

Furthermore ‘honesty’ has nothing to do with how valuable, interesting, and applicable a film is to an original text.

I think the best adaptations become intertextual and often confrontational with there source matter. I think two good examples come from Kubrick. First Lolita. Nabokov himself wrote Kubrick the script for Lolita, excited over being to re-adapt his story to a new means communication. After he finished it Kubrick told him it was an incredible script, then threw it in the garbage, and made his own film with a new script, oppositional to the novel in the most its most basic axiomatic assumptions. Watching it after reading the book is an incredible exercise in challenging readings, interpretations, and understandings of both the film and the text. Second: 2001: a space odyssey. Kubrick wrote it with Arthur C. Clarke, and yet the film has about 15 minutes of total dialog. It is in visual conversation with the book, holds many of its tenets up, crushes others, creates its own and is the best re-conceptualization of a novel I have ever seen.

So we have a situation where whether or not a film can adhere to the principals of a book has nothing to do with how closely plots resemble each other, and its in relation to the book does not require the espousing of the same themes. It is imagination, evolution, and argument.

In fact, I contend that if a perfect copy of a book could be reproduced in film that it would be absolutely worthless, except as escapism. The constructions of continuous worlds without challenge have nothing to do with thinking. Discontinuity and Struggle (which can be ameliorated within a text:see 2001) is the history of complex and valuable art.

My take on Harry Potter in lieu of my opinions above: It is facile pulp to begin with, and a perfect copy of it in film would be an unchallenging affirmation of facile pulp, the realization of a continuous multi-media conglomeration. If i had kids (10 or 19) I would want them to be challenged, and while there is nothing wrong with occasional immersing fantasy, I have no interest in promoting it on the scale of the HP empire. I think the world of HP has been turned away from whatever value and merit it may have ever had, into the quintessential example of literature as escapist consumerism.

So, lets not mention HP again. There are much more interesting book-novel adaptations we could be talking about. Like, all of them. How about children of men, that seems pertinent for some reason.

A couple side notes:
The five hour Pride and Prejudice was still not accurate. Long movies aren’t always more accurate.

The Godfather is one movie that is often considered better than the book. I’m just saying it is possible to make a movie that’s better than the book.

I think the Harry Potter movies are a poor attempt at deviating from the book in order to create a more hollywood style movie. On the other hand, Peter Jackson the Lord of the Rings director managed to stay true to the book while still creating an entertaining and interesting movie that could appeal to both movie fans and book fans. In creating the LOTR movie, the crew spent absurd amounts of time creating complex sets that followed every detail from the descriptions in the book. Things on the sets that one wouldn’t even notice when watching the movie, still were created with immense detail. Everything from the costumes to the pronounication of Elvish words were exacted in order to match what Tolkien had intended. Although there were slight plot changes and events that from the book that didn’t appear in the movie, the characters stayed true to the book and the scenes weren’t exaggerated for the sake of a good hollywood scene.

Directors need to learn to stay true to the books they are recreating while still displaying a good movie. Of course, things need to be cut from the book, but there is a right and wrong way to do this.