The creators of book-to-film adaptations are always under a lot of pressure from fans to create something that lives up to their very high expectations. People that have read a book over and over are often excited to see that it is going to be put on the big screen but are typically disappointed to see some of their favorite scenes changed or left out entirely.
This is of course true for the wildly popular Harry Potter series, whose seven books were made into eight equally popular movies. While the creators did the best they could to capture the magic of JK Rowling's books, there were bound to be scenes that were not included. The absence of these scenes may have let many fans down but some of them would have been downright impossible to include.
The Marauders Backstory
The third movie was the first to leave out large chunks of information that viewers felt were important. However, with so much going on in The Prisoner of Azkaban book, the creators were left with some difficult decisions.
Fans were disappointed to see that the film completely left out any explanation about James and his friends being the Marauders. However, with much of this being verbally given information, the creators had the choice to either give the information through flashbacks, (which would have required more casting) or verbal explanation (which would have diminished the climax of the film with too much dialogue).
The Quidditch World Cup
Quidditch scenes, whether in the books or movies, are some of the audience's favorites. However, as far as filming goes, Quidditch can be downright expensive. With flying obviously being impossible, movie producers are forced to do a lot of film magic to ensure that the sport looks convincing.
For this reason, Quidditch games are only shown in detail in the movies when they are absolutely necessary to the plot. While the Quidditch World Cup has a lot of important moments in the books, the film producers likely decided that they could make adjustments to bring the important information to other scenes, allowing them to cut the bulk of the match and stay within a realistic budget.
Barty Crouch Jr.'s Escape
The Goblet of Fire movie only explains that Barty Crouch Jr. is the son of a Ministry official who becomes a Death Eater and is imprisoned for it (much to his father's shame). However, book fans know that Barty Crouch Jr.'s father helps break him out of prison, and keeps him imprisoned at home under the imperious curse for years until he breaks free at the Quidditch World Cup and returns to his master.
The details of Crouch Jr.'s escape are complicated and are explained to Harry and his friends by Crouch's house-elf, Winky, who was also cut from the movies. Getting into these details would have required more CGI, more expensive casting, and a whole lot of talking that the film really just didn't have time for.
S.P.E.W.
House-elves are a frequent occurrence in every book after they were first introduced in Chamber of Secrets but they are very rarely shown in the movies, likely due to the complicated CGI that goes into putting them on screen.
For this reason, film watchers never get to learn about Hermione's movement, known as S.P.E.W., which is meant to free all house-elves (even against their will). Hermione develops this movement after she, Harry, and Ron visit the Hogwarts kitchen and speak with Dobby and Winky, as well as hundreds of other elves. The scene would have been a lot of time and money to include, so it was cut, as well as any subsequent scenes that include house-elves.
Harry's Rage After Sirius's Death
Sirius's death was devastating in both the books and movies, but the chapters that followed it in the books really drove home just how heartbroken Harry really was.
Unable to accept Sirius's death and the part that he played in it, an enraged Harry starts destroying Dumbledore's office, smashing instruments and trinkets. The inner dialogue that happens as he destroys everything in sight really shows just how broken Harry was feeling, but, without that peek into Harry's mind, the scene on screen might not have translated the same way. While it is understandable that the scene was impossible to adapt, movie audiences really missed out on the inner workings of Harry's grief.
All The Horcrux Memories
In the books, Dumbledore takes Harry into the pensive several more times than what is shown in the movie. Harry sees memories of Voldemort's mother (who had previously owned Slytherin's locket), Voldemort finding out about Hufflepuff's cup from an old collector, and Voldemort returning to Hogwarts to request the DADA position (which he cursed upon being turned down.)
Despite the clarity that these memories bring to Harry's search, the scenes were not included in the movies. With so many memories to get through and so little time, the creators were likely forced to pick only a couple of the most important memories to show audiences, and since even Harry Potter book fans often forget these details anyway, it may have been the right choice.
Fred's Death
The Deathly Hallows book was already split into two movies because of the extensive amount of information they needed to get across to viewers in order to wrap up the series. However, quite a few scenes are still left out that audiences had expected to see.
In the books, Harry witnesses Fred's death (which is one of the most shocking things to happen in the Harry Potter series). However, the movie only shows Fred's body after the fact, significantly decreasing the emotional impact of the situation. This felt like a poor choice to viewers, but realistically, there are so many emotionally impactful moments spread out among the last few chapters of the book, that shoving them all into a significantly shorter time frame in the movie would have been far too much.
Harry Going To The Forest Alone
One of the benefits of a written story is that audiences get to observe every thought, which can be challenging when trying to adapt these scenes that are heavy with internal dialogue.
After Snape's dying words and a journey into the pensive, Harry learns he must sacrifice himself. The movie writers had to find a way to communicate what Harry needed to do with the audience before he headed out to the forest. For this reason, instead of the silent scene where Harry heads to the forest alone without saying goodbye to any of his friends, Harry quickly explains to Hermione and Ron what must be done, and says goodbye,
Dumbledore's Explanations
At the end of all the Harry Potter books, other than Half-Blood Prince, Harry receives a lengthy explanation on the mystery that he had been mulling over all school year. Readers then get some of the best quotes of the Harry Potter series from the wise headmaster. These scenes contained a lot of important information, but in every film adaptation, the scenes are cut short.
However, at the end of Deathly Hallows Part 2, the part of this scene in which Dumbledore revealed what happened between him, his siblings, and Grindelwald is not included. This explanation, which was a deeply vulnerable moment for Dumbledore, was simply too much to squeeze in. But fans can expect the Fantastic Beasts movies to explore Dumbledore's story further.
The Final Battle
The final battle between Harry and Voldemort in the book had readers' hearts pumping the whole time. The two enemies circled around each other in the Great Hall as everyone watched on silently. Harry explains to Voldemort where he had gone wrong, and exactly why he was about to lose. But an arrogant Voldemort chose to attack anyway, leading to his death.
While this scene was perfect for the books, it may not have translated as well to the screen. The scene was much more talking and a lot less action, which can make such a climactic movie fall flat. For this reason, the scene from the book was impossible to recreate with the same effect.