Harry Potter and the Order of the Phoenix
Columbus repeatedly checked with Rowling to make sure he was getting the little details in the film correct. Kloves described the film as being "really faithful" to the book. He added some dialogue, which Rowling approved of. One of the lines originally included had to be removed after Rowling told him that it would directly contradict an event in the then-unreleased Harry Potter and the Order of the Phoenix.
Even so, as with many book to film transitions, there are differences between the plot of the film and the original source material. The first chapter of the book begins the story from the point of view of Vernon and Petunia Dursley in the days leading up to them being given Harry to look after, highlighting how Muggles react to magic. A month of Harry's summer, which includes several of Vernon's attempts to escape the constantly arriving Hogwart's letters are cut from the film. Some conflict between Harry and Draco, including their original first meeting in Madam Malkin's robe shop, is cut. The character of Piers Polkiss is cut, and some of Nicolas Flamel's role is changed around or cut altogether. Snape's potion riddle task on the way to the philosopher's stone, which Hermione solves, is also cut. Norbert is mentioned to have been taken away by Dumbledore in the film; whilst the book sees Harry and Hermione have to take him by hand to friends of Charlie Weasley. As such, the reason for the detention in the Forbidden Forest is changed, with Ron having to go with Harry, Hermione and Draco, unlike the book in which Neville has to go. The Sorting Hat's song is axed, with Scabbers appearing very little. In a visual change, Aunt Petunia and Dudley both are made brunette, having been blonde in the novel. Firenze is described in the book as being palomino with light blonde hair, although he is shown to be dark in the film.
Harry Potter and the Goblet of Fire
Numerous plots and subplots from the novel were dropped in the effort to turn the 734 page novel into a single 2 1/2 hour movie. In the movie, Sirius Black makes only one brief appearance in the embers of the Gryffindor fireplace. In the book, he not only appears in the fire, but visits Harry, Hermione, and Ron in Hogsmeade as a dog. Later, he accompanies Harry to the hospital after Harry comes out of the maze. Dobby plays a decent role in the book. Originally, it is Dobby who gives Harry the Gillyweed used in the second task. Neville does this in the movie, and Dobby is absent as a whole, as are all the house-elfs despite the fact that they have a very prominent role in the novel.
The movie leaves out the Dursley family as a whole. In the book, Harry's scar first burns in his bed at the Dursley's house and he writes to Sirius from his bedroom. In the movie, he feels his scar hurt at Ron's house and writes to Sirius from the train.
In the book, at the Quidditch World Cup, the Weasleys and Harry join the Minister of Magic in his private box, where we meet Draco's mother, Narcissa, who plays a larger role in the later books, as well as Barty Crouch's house elf. The movie shows the Death Eaters attacking everyone at the Quidditch World Cup. In the book, they are attacking 4 Muggles, in a show of power against them.
In the film, Beauxbatons is portrayed as a girls only school but in the books, several accounts of boys are mentioned in the school. Similarly, in Durmstrang, there are only boys though in the books, there are boys and girls in Durmstrang.
The movie shows Neville greatly disturbed by Mad-Eye demonstrating the Crucius Curse, but never explains why.(However it is revealed in the Pensieve Scene where Barty Crouch takes part in the incapacitation of Longbottoms) His parents were tortured into madness using this curse by Bellatrix Lestrange, as is revealed in the book.
Bartemius Crouch differs from his book counterpart; he appears more sympathetic and does not demonstrate the cruelty he does in the book, i.e. when Dumbledore relates the story of Barty Crouch Jr. to Harry, he says that it "destroyed" Crouch Sr. to send his son to Azkaban, although this can be interpreted in different ways as Crouch Sr. in the Pensieve in the film does not appear to look upon Crouch Jr. with the rage of the Crouch Sr. in the novel.