Lets face it. Very few books get acceptable movies. I came to grips with this a long time ago. I believe there are two main reasons why the movies end up being terrible. The first and the more common occurrence is the exclusion of characters or events. The second is the compression of time.
My philosophy on this topic is pretty simple, but difficult to execute unless you have pretty good self control. Don't watch the movie within six months of each time you have read the book. So, if you have read the book twice, don't watch the movie for at least a year. I didn't read the Lord of the Rings within a two years of all the movies being released and I had forgotten a lot of the stuff they left out. I wasn't super disappointed with the movies.
First, we will cover Eragon. I felt like the first book was a story of a boy and his dragon growing together. The reader really understands the relationship between the boy and the dragon. What does the movie do? The dragon learns to fly, and then comes down from the cloud and the dragon is all knowledgeable and 10Xs larger. At the end of the movie, only a week has passed (according to the characters). The book spans six months. The movie includes scenes and characters that were not in the book. Most of these included the villains.... some of these villains haven't even been introduced yet in the book series and they are three books in!
Several minor characters are also killed off, or at least you are lead to believe they are killed. These minor characters end up playing larger rolls in the later books. I've really enjoyed the Eragon book series so far. The author is a month younger than I am, and Eragon was published when he was around 19. Very awesome book for such a young author. The movie does absolutely no justice for the book.
Unfortunately, the Percy Jackson movie suffered more than Eragon. I really enjoyed the movie before I read the book. The series is about a boy who finds out that his father is a Greek God and he he goes on quests and encounters monsters and characters from Greek mythology. I loved the series on a Harry Potter level, BUT I've always enjoyed ready Greek Mythology.The book has tons of potential, but the movie just missed tons of stuff that were necessary for the sequels to make sense.
The primary villain of the series wasn't even featured in the movie. I guess on a level it is like Star Wars. You understand that the Emperor is the real bad guy, but he's never featured in person until the final movie, but he's mentioned through out the movies. You don't have any idea who the main villain is in the Percy Jackson Movie. They aren't even mentioned.
There are rivalries within the half bloods that have large parts in later books. The children of Ares aren't even shown in the movie. Ares isn't even in the movie. Ares and his children are all over the place in the later books. That's just the tip of the iceberg in terms of missing stuff.
The movie does a great job of sparking interest in the book series. I wouldn't have read the books if it wasn't for the movie. The Harry Potter series hit the mark with the movies. Maybe the success of the last few books were due to the extreme success of the earlier book movies. Maybe the Harry Potter series had a better theme for marketing or something. It's just a shame that so few great books get good movies.
The Hunger Games is getting a movie soon. The book series was just concluded a few months ago. The whole map has been laid out and I don't think the writers can mess much up unless they aren't paying attention. I think the place for error will be in the rating of the movie. If you have read the books you are aware of the plot line and some of the graphic details of character deaths. It's definitely not a PG movie and I think PG-13 can't properly portray the violence that the future society craves.
I'm really looking forward to the movie and I have high hopes for it. I am going to follow my philosophy and be lazy. I wont read the book again until the movie is released so that I wont be entirely disappointed when the movie is released.