Given my interests and line of work (i.e. the fact that I am a massive nerd), you probably won’t be surprised to know that a large number of my friends were the sort of people took great delight in pointing out (and complaining about) every little detail that was changed from the books to make the Harry Potter movies. Many of these are the same people who are now among the armies of angry monkeys kicking off after EVERY SINGLE EPISODE of Game Of Thrones.
But, what you might be surprised to learn is that I am not one of these people. Far from it, in fact…
Different Media

Harry Potter and the Philosopher’s Stone (c) Warner Bros.
You see, the thing is, books, TV, films and video games are all very different media. They employ different methods and tools to tell their stories and these techniques rarely transfer well from one medium to another.
Books, for example, tend to have a narrative voice, whether it is an external narrator entity or the voice of one of the characters. This is a very useful technique for conveying detail and background in a medium where there are no visual references. In movies, however, using a narrator, or making the audience read text on the screen, is often seen as a lazy method of exposition; movies are a much more visual medium and so it is generally considered better to SHOW people things rather than just TELLing them.
There is also a question of length. If you turn the average book, word for word, into a film they it would be longer and more tedious than even the extended versions of Lord of the Rings (which, lets remember, left out lots of stuff from the books, such as Tom Bombadil, the Barrow Downs (which is as far as I ever made it though the books!) and The Scouring of the Shire). If you turn the average movie into a video game, even taking in deleted scenes and so on, then it will only last a few hours, rather than the 10-30+ hours that you would expect a game to last.
The simple fact is that things have to be done differently when you move from one medium to another. Movies have to be padded out with extra bits and extended action scenes to turn them into a good video game, and not every scene, character or plot line from a book will make it into a the movie.
Don’t get me wrong…
Changing stuff isn’t automatically a good thing, by any means. There are some changes in the Les Misérables movie that I didn’t like, because they didn’t seem to add anything, ruined the rhyming or musical nature of the movie and I just couldn’t understand why they were made. Additionally, it sometimes seems like the changes made to Game of Thrones are purely designed to scandalise, and to get free publicity by getting everyone to talk about the show. But some changes are inevitable and it is even possible that you end up telling the story (or at least certain aspects of it) in a better way via another medium. For every good Harry Potter movie, there is an Eragon and… well, I can’t even think of a good video-game-to-movie adaptation, which only goes to show how difficult it must be!
Success Stories
Some of the biggest cross-medium success stories in recent years are the result of fairly major changes to the canon. Just look at the Marvel movies, which, though inspired by the comics, are considered to be in their own version of the universe and are therefore free to adapt the canon as they please to tell their own stories. Or the Arkham series of video games, which are set in their own version of the Batman universe and aren’t tied to specific comic book or movie storylines.
Adapting

Going Postal (c) Terry Pratchett/Mob Films
Terry Pratchett commented on this when his books were adapted into TV movies, saying that the director and writers were often being TOO respectful of the original source material. Here is a man who was clearly very much aware of how storytelling in a book and in a mini-series differ, and SHOULD differ, and who even had his own ideas about how it should be done. “I love that scene too, but if I was writing it for TV, I would have done it like this instead…”
This, I think, is a good attitude for authors to have. If you accept that your story will have to be changed, and are willing to take the advice of people who are experienced at storytelling in the new medium, then you can help out yourself in the adaptation process. This, I believe, would result in much better adaptations, whatever format they are being adapted from/to. The alternative is to let someone take your work away from you and change things without your input, which, even if the adaptation is a success, might not be something that the author is happy to have their name attached to.
Final Thoughts
I am not saying that all change is good, but sometimes it is necessary in order to tell a story in the best way possible in each particular medium. If more authors accepted this and got involved in the adaptation process, I think we would have a lot more, better-quality adaptations. It would also make things a lot easier for the creative minds behind the adaptations if the fans didn’t make a huge fuss every time Superman ditches the underpants or Peeves gets cut from the Harry Potter movies.
Sometimes, things that work in one medium do not work in another, and I think we’d all be a lot happier if we just accepted this and were prepared for it. Then we could go and see a movie based on our favourite book, without being distracted by playing spot the difference through the whole fucking thing…