It's nearly impossible to fully adapt the longest book in the Potter series to film. Many major changes had to be made. To begin with, many characters were removed, such as (in order of appearance) Mundungus Fletcher, Mr. Prentice (Muggle resident of Little Whinging), Hestia Jones, Dedalus Diggle, Walburga Black (Sirius Black's mother: her voice is heard but she is not seen), Bill Weasley, Eric Munch, Perkins, students mentioned specifically by name by the Sorting Hat, Professor Binns, Peeves, Michael Corner, Terry Boot, Dennis Creevey, Marietta Edgecombe, Anthony Goldstein, Warrington, Montague, Theodore Nott, Dilys Derwent, Broderick Bode, Miriam Strout, Madam Puddifoot, Fortescue, Griselda Marchbanks, Professor Tofty, and Williamson (Auror).
Book: The book stresses the importance of these examinations and we are able to understand more about the examinations, including Practical Examinations conducted by external professors. In the Defence Against The Dark Arts Practical Examination, Harry's professor was impressed with his abilities and urges him to show him a full corporeal Patronus for a bonus point.
Movie: Only one scene of the O.W.L. Examination is shown, in which Fred and George Weasly decided to pull their last prank on Umbridge. Nothing is mentioned about external professors as we see Umbridge being the chief examiner and nothing is mentioned about Practicals.
Book: Two students from each house are selected at their fifth year to represent the respective houses as Prefects. Ron and Hermione were selected as Gryffindor Prefects while Draco and Pansy were selected as Slytherin Prefects and so on. Harry seems relatively upset as many saw him being second to Ron. At the end, Dumbledore explains why he did not select Harry as a Prefect.
Movie: Nothing is mentioned about the Prefects. Ron and Hermione remain as normal students.
Book:The school is portrayed as a place where nothing electric is used.
Movie: Rock music (a 2005 song, the story is set in 1995) can be heard playing in the Gryffindor common room, as well as a radio, and Umbridge uses a public address system upon being made Headmistress. A decree stating that no music will be played during study hours is also passed.
Dumbledore and Harry's talk at the end
Book: After the incident at the Department of Mysteries, Dumbledore had a talk with Harry (who was on a rampage), explaining to him why he was kept in the dark for so long, explaining to him more about the prophecy and why Dumbledore seems to be avoiding eye-contact with Harry.
Movie: Dumbledore only talks to Harry in a brief scene. Harry does not go on a rampage.
Book: Harry took the Prophecy and after an intense battle at the Department of Mysteries, the Prophecy is destroyed by accident, causing failure of everyone at the scene to know what the Prophecy is all about. However, at the end, Dumbledore explains in thorough detail to Harry about the Prophecy, telling him that it was Trelawny who gave that prophecy during her interview with Dumbledore. Dumbledore also tells Harry the entire Prophecy.
Movie: Upon grabbing the Prophecy, the Prophecy immediately recites briefly. Harry and his friends hear the Prophecy and so did Lucius Malfoy as he was sneaking up on them in the shadows. Throughout the movie, the complete prophecy is neither heard nor explained by Dumbledore at all. Only a brief few sentences is heard. The origin of the Prophecy, who the Seer is, who heard the Prophecy and more is not explained by Dumbledore to Harry. The Prophecy is given to Lucius who drops it by accident.
Book: Umbridge is introduced as Undersecretary to the Minister in Harry's "hearing", after which she becomes Defense teacher, High Inquisitor, and Headmistress. A total of eight decrees are passed in this book, seen everywhere in the school.
Movie: Umbridge is first mentioned by name in Hogwarts by Dumbledore. She becomes Defense teacher, still using only theory. Her role as High Inquisitor is expanded to include blasting apart kissing students among many more things, and almost one hundred decrees are passed, all posted by the Great Hall. One of them is not in the book - students will submit to questioning about illegal societies, which will lead to the D.A.'s fall. She becomes Headmistress too. However, at the hearing she is mentioned as Madam Undersecretary though her name is not revealed.
Book: Harry sneaks into Umbridge's office to conduct an urgent check on Sirius after his Voldemort's false vision, via floo network. He meets Kreacher, who lied to Harry that Sirius is indeed at the Department of Mysteries. Harry panicked, at the same time, Umbridge returns and pulled Harry out.
Movie: Harry attempted to use the floo network but before he could enter, Umbridge returned.
Book: Harry lay in the flower bed below the open window of the Dursleys' house listening to the news on the television for anything unusual, but there's nothing unusual reported. He soon goes to the park and sits on the swing. Dudley has to lose his gang once he sees Harry. They both walk back to Number 4 Privet Drive. Dudley antagonizes Harry until he gets angry and threatens Dudley with his wand. As they pass through a dark alleyway, a chill starts brewing and Dudley suspects Harry. Two Dementors approach the cousins. Harry conjures a Patronus Charm, and Dudley becomes unwell and hears voices. Mrs. Arabella Figg rescues Harry, and explains her identity as a Squib. They escort Dudley home, Arabella expressing her anger toward a lazy wizard guard Mundungus Fletcher (who apparates for a late rescue) and explaining why Harry's on the watch.
Movie: The film opens with Harry already being at the park on the swing and there's a dull radio voice-over in the wind explaining nothing unusual. Dudley approaches Harry with his gang who start antagonizing him on the spot until Harry gets angry and threatens Dudley with his wand. A storm begins to brew with Dudley suspecting that Harry is causing it. The gang scrambles, followed by Harry and Dudley running back to number 4 Privet Drive. While passing through a tunnel, two Dementors enter and attack. Dudley slips on the icy floor and becomes unconscious. Harry performs the Patronus Charm to drive off the dementors. Arabella Figg sees the attack, and she escorts Harry and Dudley home, briefly explaining to Harry that she's been watching him on Dumbledore's request. While she obviously knows of he wizarding world, there is no mention of her being a squib.
A Peck of Owls
Book: When Harry arrives home with Dudley, both Petunia and Vernon are shocked to see their son in this state. Vernon has a row with Harry and even threatens to kick him out of the house after Dudley, who is still dazed, blames Harry, Harry explains what occurred, revealing parts of the wizarding world to them. Petunia surprises everyone by revealing that she knows that dementors are the guards of Azkaban. Her previously unrevealed knowledge of the wizarding world makes Harry curious. Many owls zoom through the Dursleys' open window, dropping off letters, one from the Ministry of Magic stating that Harry's been expelled from Hogwarts because of his use of magic, and that he is to appear at a trial. Other letters are from Sirius Black and Arthur Weasley saying to stay where he is. Vernon is about to kick out Harry when an owl arrives with a howler from Dumbledore (though Harry does not recognize it) addressed for Petunia. After being ignored by her it bellows "remember my last, Petunia". Petunia says that they must let Harry stay, but gives no further explanation.
Movie: When Harry arrives home with Dudley, Petunia is shocked to see her son at this state, Vernon soon realizes the situation and begins fuming at Harry when Dudley, who is still dazed from attack, points to Harry as the cause. Harry does not mention the dementors or the wizarding world, Instead of having a nervous-breakdown at her son's condition, Petunia seems relatively calm, hinting to the readers she could have known something. A single owl from the ministry zooms through the window with a letter, which takes the form of a woman's face and informs Harry that he's been expelled from Hogwarts because of his use of magic and is to be present at a trial. No howler to Petunia is sent from Dumbledore,or letters sent from Arthur Weasley and Sirius Black.
Book: After Harry prys at Petunia knowledge of magic world, he is sent to his room. Harry sends a letter to Sirius about the ordeal. Days later, the Dursley leave to recieve a so-called prize they won. At their absence, noises are heard downstairs, Harry investigates only to find a large group of people at the doorway of the Dursleys, two of which he knows: Mad-Eye Moody (who Harry is a little uncomfortable to see after his 4th years experience) and Remus Lupin who have come to take Harry to Grimmauld Place the Headquarters of the Order of the Phoenix. Two other prominent characters of the large group were Kingsley Shacklebolt and Nymphadora Tonks who Harry does not know. Tonks helps Harry pack and reveals her Metamorphamagus powers to him. After questioning about the trial, Harry is enchanted by Moody to become partially invisible. They all depart on brooms, with Harry to follow Tonks. Moody stresses to fly very high above the cover of clouds. On the flight some discussion as to what route they should take happens.
Movie: After the Dursleys leave to get Dudley help (making it secret as possible), Harry goes to isolate himself in his room. That night after dreaming a strange dream, Harry sees that his locked door is unlocking. The door opens to reveal a group of three people: Mad-Eye Moody, Kingsley Shacklebolt and Nymphadora Tonks who have come to take Harry to Grimmauld Place, the Headquarters of the Order of the Phoenix. The scene cuts to Harry and the Advance Guard (which only consists of 5 people) exiting Number 4, Privet Drive. After Harry questions on the trial and Tonks is introduced to the audience along with her powers, the party departs on brooms, Harry to follow Tonks. They are somewhat careless with their flying, going around boats and through the city.
Number Twelve, Grimmauld Place
Book: At their destination, Moody gives a note written by Dumbledore with the address of Grimmauld Place to Harry, who reads it and then a door to Number Twelve appears between Eleven and Thirteen. Lupin opens the door with his wand and they enter. Mrs Weasley greets Harry and tells him the meeting of the Order has started so he can go up to see Hermione and Ron. Once he meets them he expresses his anger on how he's been isolated over the summer, and they tell him they've swore to Dumbledore to not inform Harry on any news. Fred & George apparate in and they attempt to use the Extendable Ears to spy on the Order's meeting but are foiled by an Imperturbable Charm placed on the kitchen door by Mrs Weasley. On the descent for dinner, the portrait of Sirius' mother curses the visitors.
Movie: At their destination, Moody taps his staff on the ground causing Number Eleven and Thirteen to split apart revealing Twelve. They enter without unlocking the doors. Mrs. Weasley greets Harry and tells him the meeting of the Order has started so he can go up to see Hermione and Ron. Upon his ascent, he spots Kreacher. Once he meets Hermione and Ron he expresses his feelings on the dementor attack and his expulsion, and they tell him they've swore to Dumbledore to not inform Harry on any news. Fred & George apparate in and they use the Extendable Ears to spy on the Order's meeting, but Crookshanks foils their eavesdropping.
The Battle of the Department of Mysteries
Book: Harry, Ron, Hermione, Neville, Ginny and Luna are chased from the room containing the prophecies. While attempting to escape, Hermione sustains serious injury and becomes incapacitated. In the confusion, Harry and Neville are then pursued to the entrance of the department by the Death Eaters. Once there, they find Ron, who was hit by a spell, as well as Ginny, who had a broken ankle. Harry and Neville run with Ron into the Thought Room of the department, which is filled with rows of green aquariums, each containing a brain. Ron is attacked by the brains and Luna is knocked unconscious. Harry and Neville are then chased into the Death Chamber which holds the archway with the black veil. Harry is confronted by Lucius Malfoy and instructed to hand over the prophecy. Reinforcements arrive in the form of Dumbledore and the rest of the Order of the Phoenix, and in the ensuing battle, the prophecy is destroyed accidently by Neville while he and Harry are attempting to escape, and Sirius is murdered by his cousin Bellatrix Lestrange with a spell, which pushes him through the Veil. While Voldemort and Dumbledore duel, Voldemort fires off many killing curse at Dumbledore who brings the golden statues to life to protect himself. Also, Fawkes protects Harry.
Movie:In the film, the battle is condensed into three rooms (the room holding the prophecies, the Death Chamber and the entrance hall) and none of the characters sustain any serious injuries. Luna Lovegood uses the Levicorpus spell against a Death Eater even though this spell, invented by Severus Snape as a student at Hogwarts, is not discovered until the sixth book when Harry is given Snape's old potion book. The portion in the Thought Room is omitted. Sirius gets hit by the killing curse then falls backwards through the Veil and disappears. In the battle between Dumbledore and Lord Voldemort, not a single killing curse is made nor the statues never come to life. Fawkes was not present.