Books and movies are fundamentally different mediums: books are meant to be read over long periods of time, days, weeks, or even months depending on the reader. Movies however, are meant to be watched in one sitting, generally between ninety minutes and three hours. The key problem here is pacing. Books have a tendency to have a lot happen in them, just look at Harry Potter, the first book was a mere three hundred pages, give or take, and it took nearly three hours to make into a movie. That was the first Harry Potter movie and, aside from one scene, the movie was nearly identical to the book. Let me say that again, three hours to tell three hundred pages, and the movie wasn't great either. Due to it's adherence to the book the first Harry Potter movie was riddled with awkward dialogue and over acting, and for it's extend length the movie didn't even even have all that grand of a plot.
Lets compare that with the fifth Harry Potter movie, which removed a considerable portion of the book and added in scenes to fill the gaps with more important, and better presented scenes. All told the fifth movie was considerably better as a movie than the first. Yes it still had about a three hour running time, but the story it got through in that three hours was nearly eight hundred pages, and because of the significant editing the movie was presented more like a movie than a book that was just filmed.
Another good example of a movie being made into a book correctly is the Lord of the Rings which is probably the greatest piece of fantasy fiction ever put to film. And, yes, Lord of the Rings stuck to the books quite well, save for a few moved around scenes and some editing for theaters (the edited content, which was quite significant, was later added in anyway in the Extended Editions, which is a must own for any movie enthusiasts collection), but what it got right was pacing. Peter Jackson knew that if he rushed through LotR with the same recklessness as was put into the first few Harry Potter movies (which, coincidentally, premiered about the same time as LotR) then they would have the same problems, particularly awkward dialogue and overacting, as well as a hollowness to the story. So, what did he do right? Pacing. It has become sort of a joke that the Lord of the Rings movies are incredibly long, and they absolutely are, but for very good reason: Peter Jackson wanted to make sure the audience had adequate time to get to know all of the characters. Every single character in the movie is introduced deliberately and effectively. Remember the first time you meet Aragorn and he's this mysterious ranger calling Strider, that you don't know if he's friend or foe? Didn't that make you incredibly interested in the character? And then he saves the hobbits lives and leads them to Rivendell! Aragorn was an awesome character, and one of the most well introduced characters in the series.
Now we've reached the point of characterization and this is the main reason for the discrepancy between those who have read the book and those who haven't when watching a movie. PEOPLE WHO HAVE READ THE BOOK ALREADY HAVE AN EMOTIONAL ATTACHMENT TO THE CHARACTERS IN THE BOOK! Comparatively, those who are watching a movie as an introduction to the story do not have that emotional attachment to the characters. So, why is this important? Those who have read the book only want to see the scenes acted out, they already know what's going on in each characters mind as well as what their hopes and dreams are and etc... They have an emotional attachment to the characters so the movie doesn't have to try and build it up. Conversely when someone is new to the story and a movie just replays the scenes from the book without giving the audience a way to really get to know the characters, the audience then has no reason to care about the people on screen, or any danger they might be in. This makes movies flat and boring for anyone who hasn't read the book.
Finally, what is the best way to convey a book in movie form, despite the fact that they're entirely different mediums? It's simple, though not easy. Step one is to read the book and decide what the theme and tone of the book are. Try and figure out what the book is trying to convey to the reader. Why was this book written, essentially. Then make damn sure your movie delivers that to the audience. Step two is to make sure the characters are likable and have human depth. Flat, one dimensional characters can ruin any movie, no matter how good the plot is. And finally, step three is to get the events in order. Add what you can from the book but only as long as it helps the movie say what it needs to say. Never sacrifice pacing for more scenes from the book, pacing is important and it can make or break a movie. Don't be afraid to make up your own scenes, if they serve the purpose better than anything in the book they add them and the movie will be better for it.
The key point to remember is that a movie is a movie and a book is a book. A movie should be taken on its own merits and not on how close to the book it is, so make sure your movie is good for everyone watching, even the ones who haven't read the book.
