The Gryffindor dormitory chill
The books do mention house dormitories. But they don’t write about the downtime for students that we see in the movies. Particularly, in the Prisoner of Azkaban when the gang is trying out different sweets that make you produce animal sounds and effects.
This scene is short and doesn’t have any significant meaning attached to it at first glance. But it is important as it adds to the realism of the movie and the characters. It shows them as just teenagers, which we might sometimes forget.
Professor McGonagall giving Ron a dancing lesson
While Professor McGonagall does teach the students some proper etiquette in the books, this scene is only in the movie. It is when she picks Ron to demonstrate to the class how to properly ask a girl’s hand for a dance, and how to perform the basic waltz.
The scene is again quite warm because it shows our lads as just being boys, which is so often not the case. They are usually fighting for their lives and doing things way out of their league. The audience can sometimes forget that they are just teenagers, easily embarrassed and amused.
Hedwig died during the Battle of the Seven Potters both in the book and in the movie. In Harry Potter and the Deathly Hallows book, his pet owl dies in her cage. She was aboard the motorcycle with Harry, which Hagrid drove.
Snape killed the bird still maintaining his cover as a loyal Death Eater. But, in the movie, we see Hedwig protecting Harry from Avada Kedavra with its body. The curse killed her and this is how Voldemort knew which Harry was the real one.
Locking Dudley into the snake pit
There’s much more punishment for the Dursleys in the movies in general. Locking up Dudley in the Snake Pit at the zoo was brilliant. And Vernon falling out of his window is also not in the book. These scenes undoubtedly bring everyone huge satisfaction.
Neville giving Harry the Gillyweed
When Harry gets to the second round of the Triwizard Tournament, he is faced with a problem. He can’t breathe underwater, and he doesn’t know the charms or tricks the others use. However, this is where the book and the movie, Harry Potter and the Goblet of Fire, are very different.
In the book, Dobby helps Harry. He steals the gillyweed from Snape, or from his potions class storage more accurately. But, giving Nevile a passion for potion making and giving him this role in the movie was a good move. It established the character more and it probably saved the production a lot on CGI effects.