This is probably one of my favorite book of the series, loosing, obviously, to the Half Blood Prince.
Let’s start from the beginning: Harry Potter, aware of the Restrictions of Underage Wizards for the Use of Magic, does not preform the Lumos or Lumos Maxima spell while reading and doing his homework. Instead, he uses a simple torch to do his homework without his uncle knowing and hides all his stuff under a floorboard.
On his birthday, Harry receives a Pocket Sneakoscope from Ron, a Broomstick Servicing Kit from Hermione and The Monster Book of Monsters from Hagrid. Ron’s letter also informs him that Mr Weasley got a money prize, allowing him to have a new wand, and that they all went to Egypt to visit Bill: “Harry scanned the moving photograph, and a grin spread across his face as he saw all nine of the Weasleys waving furiously at him…Right in the middle of the picture was Ron, tall and gangling, with his pet rat, Scabbers, on his shoulder…“.
Aunt Marge comes to Privet Drive to stay for a few days. During this time, Harry promises he’ll be on his best behavior, in order to get his uncle to sign the permissions form that allows him to visit Hogsmeade during the school year. He blows her up after she’s already been there for a few days and after enduring her treatment and horrible manners for a while. After being blown up, she doesn’t wander off, instead she floats up to the ceiling.
After boarding the Knight Bus, Harry claims to be Neville Longbottom, thinking that if the Ministry finds him, they might arrest him for performing magic outside of school.
Harry spends a good amount of time in The Leaky Cauldron! He visits Diagon Alley daily, where he sits eating ice cream and doing his homework. When he finally meets up with Ron and Hermione, the three of them go to the Magical Menagerie, in order for Ron to get a “Rat Tonic”, given that Scabbers isn’t looking that well, and Hermione has her mind set on getting an owl, ending up buying Crookshanks instead.
During the Dementor’s attack in the Hogwarts Express, Ginny and Neville are in the same compartment as Harry, Ron, Hermione and Lupin.
When they get to the Castle, Professor McGonagall calls Harry and Hermione, so that Harry can be examined by Madam Pomfrey and so that Hermione can receive her Time-Turner in private. This causes both of them to miss the sorting ceremony.
During their first class of Care of Magical Creatures, there are a lot of Hippogriffs around, the movie however, only shows Buckbeak. Also, when Harry rides on Buckbeak, they only fly once around the paddock.
Given Malfoy’s injury, Snape makes Ron and Harry assist Malfoy in the preparation of his potion during class.
In the book, Peeves describe Sirius Black’s attack on the Fat Lady’s portrait after learning about this from her when he saw her in a portrait on the fourth floor, in the film it was the Fat Lady herself that described the attack from a nearby portrait.
After the attack, the Fat Lady portrait in the Gryffindor house entrance is replaced by one of Sir Cadogan.
During the Gryffindor-Hufflepuff Quidditch match, Hermione casts the Impervious spell on Harry’s glasses in order for him to able to see. Harry falls from his broom and Cedric catches the Snitch, after which he wants a rematch, saying that he did not see Harry fall and for this their victory isn’t fair, but everyone agrees that they won fairly.
Such a pity that they didn’t explain the Marauder’s Map origin properly on the film! They didn’t explain that they all became Animagus or why they did it or the writing of the map… pity!
Harry actually goes to Hogsmeade twice through the secret passage. The first time he goes there, without his Invisibility Cloak, he listens to the conversation between Fudge, Madame Rosmerta, Hagrid and McGonagall about Sirius Black, by hiding under Ron and Hermione’s table (so all three of them listen to the story). After learning that Sirius is his Godfather, Harry becomes angry and depressed, also due to many people knowing it and not telling him.
Harry receives his new Firebolt for Christmas, right after his Nimbus 2000 was destroyed. Not knowing who sent it, Hermione tells Professor McGonagall, who confiscates the broom for a long time in order to make sure that it’s not jinxed. Eventually, Sirius reveals that he’s the one who sent the broom, to make up for 13 years of gifts that he wasn’t able to send.
The Christmas feast was held with only 13 people.
When Buckbeak’s trial is settled, Hermione and Ron help Hagrid with his defense.
Ron and Harry stop talking to Hermione for a long time, Ron believes that her cat killed Scabbers and Harry is mad because she told on him about the Firebolt. Hagrid is the one who sets them straight and then they all make up.
In the book, during his first anti-dementor lesson, Harry has two failed attempts to ward off the Boggart/Dementor. Also, the memory that he uses to make the Patronus work is the memory of how happy he was that he would be leaving the Dursleys, not one of his parents like in the movie.
Sirius gets in the Gryffindor dorms at night and tries to kill Wormtail, who’s sleeping in Ron’s bed. He manages to get in because Neville loses the paper he has of the passwords to the common room.
During the Gryffindor-Ravenclaw Quidditch game, Marcus Flint, Crabbe, Goyle and Malfoy dress up as Dementors do throw off Harry, but he produces a Patronus and makes them fall of. Also, this game introduces Cho Chang and Harry is immediately attracted to her.
In his second trip to Hogsmeade, Harry takes his Invisibility Cloak and throws mud-balls to Draco and his friends. During this, however, Harry’s cloak slips and Draco sees his head floating midair, learning about Harry’s unauthorized visit to the village. When this happens, Harry and Ron are alone, given that they weren’t on speaking terms with Hermione.
Snape catches Harry with the map in the hall right after he climbed up the One-Eyed Witch’s hump after being seen by Malfoy in Hogsmeade. When Snape is insulted by the Marauder’s Map, each Marauder says something different: “Mr. Moony presents his compliments to Professor Snape and begs him to keep his abnormally large nose out of other people’s business. Mr. Prongs agrees with Mr. Moony and would like to add that Professor Snape is an ugly git. Mr. Padfoot would like to register his astonishment that an idiot like that ever became a Professor. Mr. Wormtail bids Professor Snape good day, and advises him to wash his hair, the slime-ball.“
Also, Harry never sees Peter Pettigrew’s name on the map!
In the book Hermione slaps Malfoy (the punch in the nose in the movie works fine as well =)).
The Gryffindor-Slytherin Quidditch match where Gryffindor defeats Slytherin and wins the Quidditch Cup was omitted.
Professor Trelawney’s Prediction happens during Harry’s examination and long after Hermione has left the class.
Crookshanks’ role is almost entirely removed. His friendship with Padfoot, him stealing the passwords and giving them to Sirius and his role in the Shrieking Shack were completely cut.
Let’s make something very clear: in the books, Ron is generally depicted as being cool headed, brave and heroic under stress, while Hermione was depicted as being nervous, terrified and frightened. Somehow, in the movies, their roles got reversed.
Sirius, a.k.a. Padfoot, grabs Ron by the arm and Ron’s leg is broken by the Whomping Willow.
The story regarding the origin of the Shrieking Shack and the planting of the Whomping Willow at Hogwarts is omitted, as is most of the Marauder’s story.
In the book, Snape holds a grudge against all the Marauders, given that Sirius almost killed him on a prank and James saved his life. Given this, when Snape confronts Sirius and Lupin he’s agitated, angry, even bounding and gagging Lupin and threatening Hermione with expulsion. After this, Ron, Hermione and Harry try to disarm Snape, causing him to hit against the wall and go unconscious (in the movie only Harry disarms Snape, and Ron and Hermione are rather shocked by this).
Snape had to be suspended in mid-air by magic to be brought out of the Shrieking Shack and he was not there to protect the trio when Lupin transformed into a werewolf. Snape gains consciousness only after Sirius and Harry become unconscious because of the Dementors.
Hermione is with Harry and Sirius when they are attacked by the Dementors.
In the film, when Harry and Hermione go back in time, Hermione throws an ammonites fossil at Harry to get his attention, so he can see Fudge and Macnair coming to Hagrid’s hut. Hermione later howls like a wolf to call Lupin away from fighting Padfoot, in turn causing him to come after them instead, at which point they are rescued by Buckbeak. In the novel, neither of these two events occurs.
In the book, Harry is alone when saving himself, Hermione, and Sirius from the Dementors due to Hermione and Buckbeak being in Hagrid’s empty cabin when they went there to hide after Lupin transformed to avoid being attacked and Harry went out to scout ahead.
Sirius sends a letter to Harry through a little owl, in which he gives Harry permission for his Hogsmeade visits the following year. He also informs that the little owl is for Ron, to replace Scabbers.
When Harry gets out of the Hogwarts Express and meets his uncle, he tells him that he has a Godfather, who’s a convicted criminal and has escaped prison.