I know plenty of people who enjoy a film so much that they later read the book (or vice versa). However, I started reading a book, noticed the film was on TV and am watching it right now (while I'm still partway through the book).
The film has a completely different tone than the book (and so far, even the story is different). Am I making a mistake?
The film has a completely different tone than the book (and so far, even the story is different). Am I making a mistake?

The book 'The Godfather' is a lot better than the movie. The movie is great, but the book is even better. The movie goes along with the book very closely but there are parts left out or shortened, and the book is a lot more graphic.
Now I can't get excited watching that movie without a leg bursting through the ceiling.
I'd much rather read the book first and imagine all the characters, settings nuances etc, for myself than have someone elses 'vision' influence my mind first.
I'd much rather finish the book, then AFTERWARDS watch the movie as a "summary".
If i was in the middle of reading and then knew the reminder of whats going to happen from the movie, i don't know if i would bring myself to finish reading. I like to be surprised.
Also, imagining stuff myself. EVEN if the author takes part in the creation(such as was with Game of Thrones). I know Daenerys had to be older for some reasons, but i imagined her quite a bit differently, same with Aria, Joffrey, Theon.... Tyrion, Tywin and Sansa were spot-on though for me. :)
Prime example being Eragon. The book, the entire series really, was amazing. The movie, however, was one of the biggest "book rapings" I have ever seen!
Thanks all. You all called it the right way.
And as for my own tastes.. it depends. Most times the book is always better, but there have been times when I've done the movie first and then the book just so the movie wouldn't be ruined for me.
And sometimes I've gotten books because of the movies. For example - Act Of Valor. I saw it in the theatre, picked up the book (iBook) recently, and even though I'm reading the book I watched it last night as well. Although in that case the movie version was really top-notch and I know most of the "mistakes" are on purpose for security reasons.
The reason being that the book is usually the original story. The film, being a shortened adaptation, often has many good parts omitted.
I also often get the impression that the director didn't fully understand the author's intentions and somehow makes something completely different. There's also often partial re-writes of the story in order to make it fit in just 2 hours or less.
That being said, I'm often surprised by the extant that they manage to fuck up a good story.
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Personally, I don't have time to read all the books I want to, so I watch the film instead.
If the film is very good, I will go read the book, knowing that it must be a true masterpiece if it still retains some brilliance after the film industry has digested it and shat it out.
I can understand how books which are very descriptive or factual could be filmed easily (given enough film for it all), but things with an internal narrative are harder (how do you portray someone's thoughts on screen, apart from with verbal narration?)
Some of the books I love are loved as much for the beauty of their language as the story. It's another one that's tough to translate. After all that you have pressures outside of the director on editing and keeping something down to 96 minutes (I guess it's why there are so many director's cuts... sadly a film has to make us love it in precis first as nobody has the patience to see if it's worthwhile even as a longer film, never mind a book).
I have seen a few films that are on par with their respective books, but usually, for better books, it's harder to make a good film.
Sometimes I think it would be better to adapt certain books into a series, but the market for 90 minute movies is surely much bigger.
A clockwork orange. I'm yet to read the book but my mother read it awhile ago. Apparently, the original book was set somewhere around the 50's/60's on EARTH! Yet, the film appeared to be some strange futuristic world opposite of earth. At the ending, in the book they didn't change his love for music, but yet they did in the film. Why change something that makes a person "human"?
The Harry Potter series. The first few films were excellent, but the quality went on a decline afterwards. The third to me was the best film, but a crucial mistake was made! They wore Muggle clothing instead of their School Robes and this trend unfortunately carried on into the other films. Both part one and two of The Deathly Hollows was severely butchered. >_<!
The lord of the rings trilogy remained truthful and perfect on both the live-action and animated remakes, but with a few changes.
Studio's Ghibli's remake of Howls Moving Castle was also perfect so, the outcome of remakes really depends on many different factors. I still think it's better to read the book first instead of judging it due to the film.