Warning: Here there be spoilers....
J. K. Rowling's Harry Potter series could well be the most successful series of books in publishing history. I must admit that I am among the millions who have come under their spell. Of course, as with any successful series of books, there were the inevitable film adaptations. And, as might be expected, like most film adaptations they have sometimes met with mixed reactions from fans of the books.
Indeed, as usually happens with film adaptations, there have sometimes been significant changes in the movies from the books. The most obvious of these have been omissions of various characters, subplots, incidents, and so on from the novels. Even in the earliest of the films, which tended to be much more loyal to the novels than the later ones, there were sometimes things in the novels that simply do not appear in the films. While they appear in the book Harry Potter and the Sorcerer's Stone (that's what it's called here in America--the original British title of both the film and the book is Harry Potter and the Philosopher's Stone), Peeves, Professor Binns, and Pansy Parkinson do not appear in the film (a scene with Peeves was filmed, but has not yet been released). In Harry Potter and the Chamber of Secrets, Nearly Headless Nick's deathday party is ommitted entirely.
Of course, as the series has progressed the novels have grown in length. Because their sheer length prevent every single thing in the later books from being included in the movies, much more from those novels have been omitted. In Harry Potter and the Prisoner of Azkaban much of the backstory was even omitted. We are never told that James Potter (Harry's father) was an animagus (a mage capable of changing into an animal). The explanations behind Moony, Padfoot, Prongs, and Wormtail are not included in the film. As to other omissions, they are really too many to include all of them here. Neither Oliver Wood nor Cho Chang appear in the movie. The Quidditch House Cup game is omitted. The movie version of Harry Potter and the Goblet of Fire omitted even more material from the book. The House Elves do not appear in the movie and hence neither does the entire subplot involving Hermione's organisation (consisting entirely of, well, herself) The Society for the Promotion of Elfish Welfare (S.P.E.W.). The Dursleys do not appear in the film at all, so that the movie begins with Harry already at the Burrow (the home of his friend Ron Weasley and his family). The fact that Karkaroff, headmaster of the Durmstrang school of magic, is a Death Eater is excluded from the film. As a result, he is never shown fleeing Hogwarts once Voldemort has returned.
As often happens in film adaptations of books, not only are things omitted from the books, but often things are changed as well. Even in the first movie, which was more loyal for the most part than most of the films, changes were made. For instance, Harry does not find out that Voldemort killed his parents until after he buys his wand. Just as there are more omissions in the later films, there are also more changes from the books. In the film version of Harry Potter and the Prisoner of Azkaban, the sexual tension between Ron and Hermione is much more obvious than it was in the book. Indeed, at one point Ron and Hermione unintentionally hold hands! In the book Harry receives his Firebolt broom as a gift from Sirius Black at Yule. In the movie it appears at the very end of the film. The following film, Harry Potter and the Goblet of Fire, saw many alterations as well. In the book, when Harry discovers Barty Crouch Sr. (by then quite mad), he is walking in the forrest with Viktor Krum. In the movie, he is walking with Ron, Hermione, and Hagrid. Similarly, although it is made clear in the book, it is never said in the movie that Crouch had been placed under the Imperius Curse (one of the Unforgivable Curses in the Harry Potter mythos, whereby the caster has complete control of the victim's mind).
Naturally, these omissions and changes have caused some controversy among Harry Potter fans. There are purists who want as much from the books included as possible, and there are perhaps even more fans who do not want to see anything changed from the books. My own thought on the omissions is that they are a necessary evil. Particularly with the later books, it would be impossible to include everything without the movie approaching six hours or more in length. For that reason the filmmakers must omit some subplots, characters, and so on. As to changes from the book, I suspect the filmmakers are probably altering things so that they simply play better on film. Let's face it, books and movies are different media. What works very well in a book may not work very well in a movie. While I do have some objections to some of the changes made from the books, I cannot say that I have any objection to alterations from the books in general.
As to the films themselves, I must say that I have enjoyed all of them. Indeed, I think that they have gotten better with each succeeding one. And while they do tend to stray from the letter of the books moreso than the earlier films, I do think that the later movies are more loyal to the spirit of the novels. Both Harry Potter and the Prisoner of Azkaban and Harry Potter and the Goblet of Fire are much darker movies than the first two. And Harry Potter and the Goblet of Fire is definitely the most English of films. Much of the British slang found in the book even made its way to the movie! While I love all the films (just as I love all of the books), Harry Potter and the Goblet of Fire is my favourite movie of the series.
Of course, much of the reason I love the films is that they have cast the movies very, very well. Daniel Radcliffe, Rupert Grint, and Emma Watson are perfect as Harry, Ron, and Hermione. And there could not have been a better Dumbledore than the late, great Richard Harris. Even the secondary characters, such as Professors McGonagall (played by Maggie Smith) and Snape (played by Alan Rickman) were cast perfectly.
The film adaptation of Harry Potter and the Order of the Phoenix is due to be released next year. It will be interesting to see how loyal it is to the book and to see if it is even better than the first four movies. Regardless, given how much I love both the Harry Potter books and movies, I will definitely go see it.