History AdaptationExplanationExtrication / HarryPotter

** Without the book's explanation that he's secretly a Squib — descended from a magical family, but unable to do magic himself — it's left a mystery why Filch's cat, of all things, would be the basilisk's first victim in the movie, nor should he have had an immediate reason to think Harry was responsible. The scene establishing Filch as a Squib was originally filmed, but ended up being cut from the final product.

** The circumstances surrounding the death of Barty Crouch, Sr. differ greatly between the book and film. In the book, he shows up half-mad in the woods near Hogwarts (having spent days fighting off the Imperius Curse, as explained later), but disappears when Harry runs to go get help, later revealed to have been murdered by Crouch, Jr. The film changes it so that Crouch appears to see through his son's disguise as Moody, but is found dead in the woods in the very next scene, and then forgotten about in the scene afterward. There's definitely the implication that Crouch, Jr. killed him, but it isn't directly stated, nor is he confirmed to have known anything about Voldemort's plans to begin with.

** Without the book’s explanation that he’s secretly a Squib — descended from a magical family, but unable to do magic himself — it’s left a mystery why Filch’s cat, of all things, would be the basilisk’s first victim in the movie, nor should he have had an immediate reason to think Harry was responsible. The scene establishing Filch as a Squib was originally filmed, but ended up being cut from the final product.

** Without the book’s explanation that he’s secretly a Squib — descended from a magical family, but unable to do magic himself — it’s left a mystery why Filch’s cat, of all things, would be the basilisk’s first victim in the movie. The scene establishing Filch as a Squib was originally filmed, but ended up being cut from the final product.

** Less flagrant than other examples of this in the films, but no mention is made of Smeltings Academy, so Dudley just happens to be wearing [[http://vignette1.wikia.nocookie.net/harrypotter/images/9/9f/Dudley.jpg/revision/latest?cb=20150102172850 a maroon tailcoat, orange knickerbockers, and a straw boater hat]] for no apparent reason. However, a deleted scene shows that he was wearing those clothes because they were his new Smeltings school uniform and was posing for a photograph in them.

** A minor inversion that’s carried into later installments: In the books, students at Hogwarts tend to be referred to according to their houses (“a first-year Gryffindor” or “a third-year Ravenclaw” and so forth), even though the books are meant to be told from Harry’s perspective, and Harry isn’t in a position to be keeping track of which house each and every student belongs to. In the films, the Hogwarts robes are given color-coded scarves and neckties corresponding to the four houses, meaning anyone can tell which one a student belongs to just by looking at them.

** In an inversion, the film explains why Harry couldn't use Parseltongue to appease the basilisk when he fought it in the chamber -- in the book, he never even tried for no adequately explored reason, but in the film, Riddle immediately dissuades him from doing so by telling him that the basilisk will only obey the Heir of Slytherin.

** Another inversion: in the book, Harry casting the Patronus he'd initially attributed to his father was the result of a random EurekaMoment on his part. The film adds in other cases of future characters influencing past events (Hermione throwing the rocks into Hagrid's hut and howling to distract werewolf!Lupin), so that there was a solid basis for his realization there.

** Dumbledore's backstory is broadly hinted at -- it's said that his father killed some Muggles, he was once friends with Grindelwald, and he has a brother who isn't very fond of him and a sister who died young -- but it's never explained how any of these events tie together. By extension, the film leaves out an explanation of events from ''Half-Blood Prince'', specifically Dumbledore's reaction to drinking the potion on the journey to recover Slytherin's locket, which Harry recognized in hindsight was due to him reliving the events that led to his sister's death. It was this realization that led to Harry's and Aberforth's faith in Albus being restored, whereas in the movie Harry's wavering trust in him never comes up and Aberforth's is seemingly resolved on its own and without incident. Somewhat rectified by the ''FantasticBeasts'' films, which have a younger Dumbledore mention the loss of his sister to another character with the implication that later installments will explore this more.

** Mr. Weasley is shown taking Harry to the Ministry of Magic by way of an elevator disguised as a phone booth. This seems to be an odd and impractical way to get there, especially since as soon as they step out of the elevator, people are shown [[TeleportersAndTransporters Apparating]] and using Floo Powder to arrive at the Ministry. In the book, it's explained that Mr. Weasley thought, since Harry is up on charges of illegally using magic, arriving at the Ministry in as non-magical a way as possible would give a good first impression.

** In an inversion, the film explains why Harry couldn't use Parseltongue to appease the basilisk when he fought it in the chamber -- in the book, he never even tried for no adequately explored reason, but when he does try it in the film, Riddle immediately tells him that as the Heir of Slytherin, the beast only obeys his commands.

** The movies never explain that Sirius willed his house -- and by extension Kreacher -- to Harry. So there's no explanation in ''Deathly Hallows Part 1'' when Kreacher obeys Harry's every command (despite his clear distaste for Ron and Hermione). Of course, if the director and screenwriter of ''[[Film/HarryPotterAndTheOrderOfThePhoenix Order of the Phoenix]]'' had cut Kreacher entirely as they originally intended, it would have made the scene even ''more'' incomprehensible to people unfamiliar with the books ... J.K. Rowling herself intervened during the development of the fifth film, cryptically telling the director and screenwriter -- and by extension, readers -- that Kreacher would be pivotal to the at-the-time-unreleased seventh book and needed to be kept in the movie.

** In an interesting inversion of this trope, the film explains why Harry never tried using Parseltongue to appease the basilisk when he fought it in the chamber, as Riddle taunts him with the knowledge that as the Heir of Slytherin, the beast only obeys his commands.

** The books made no secret that Hermione was not exactly one to upkeep her appearance on a conventional level, often remarking on her bushy hair and long teeth; this is a long set-up for the Yule Ball where she cleans up to the point of being unrecognizable at first glance. The movies dispensed with Hermione's eccentric look after the first film; Emma Watson initially started out with bushy hair and false long teeth (the latter removed for good after shooting one scene with them, the final scene on the train platform where they are visible) but retained a more natural appearance the rest of the series. This has the unintended consequence of taking the intended WhamShot as it was in the novel out of her appearance at the Yule Ball, where it just looks like Hermione as usual in a nice dress.

** While Kreacher made at least a token appearance in the fifth film, Dobby was omitted from everything between ''Chamber of Secrets'' and ''Deathly Hallows''. The latter film added a scene to reintroduce him, but it's never explained where he was or what he was doing in the interim, why he suddenly reappeared when he did, or how Aberforth was in contact with him to send him to the rescue in the critical moment, and his relationship with Harry is also downplayed. In the book, Dobby had been working at Hogwarts since ''Goblet of Fire'', making his appearance less sudden and explaining his connection to Aberforth.
** A scene explaining the Taboo (Ron mentions hearing about it in the Ministry) was cut from ''Deathly Hallows Part 1'', so it's never explained despite its effects showing up in two plot-critical moments (the book-verbatim Death Eater attack in the café, and a new change to the Lovegood house scene where Xenophilius says Voldemort's name to summon Death Eaters), making them seem like DiabolusExMachina rather than a jinx. It also saves the writers some trouble, because through the movies it was never firmly established that so many wizards are afraid of saying Voldemort's name, when it was an important part of the books.

** Dumbledore's backstory is broadly hinted at -- it's said that his father killed some Muggles, he was once friends with Grindelwald, and he has a brother who isn't very fond of him and a sister who died young -- but it's never explained how any of these events tie together. By extension, the film leaves out an explanation of events from ''Half-Blood Prince'', specifically Dumbledore's reaction to drinking the potion on the journey to recover Slytherin's locket, which Harry recognized in hindsight was due to him reliving the events that led to his sister's death. It was this realization that led to Harry's and Aberforth's faith in Albus being restored, whereas in the movie Harry's wavering trust in him never comes up and Aberforth's is seemingly resolved on its own and without incident.

** The books made no secret that Hermione was not exactly one to upkeep her appearance on a conventional level, often remarking on her bushy hair and long teeth; this is a long set-up for the Yule Ball where she cleans up to the point of being unrecognizable at first glance. The movies dispensed with Hermione's eccentric look after the first film; Emma Watson initially started out with bushy hair and false long teeth (the latter removed for good after shooting one scene with them, the final scene on the train platform where they are visible) and retains her natural appearance the rest of the series. This has the unintended consequence of taking the intended WhamShot as it was in the novel out of her appearance at the Yule Ball, where it just looks like Hermione as usual in a nice dress.

** Mr. Weasley is shown taking Harry to the Ministry of Magic by way of an elevator disguised as a phone booth. This seems to be an odd and impractical way to get there, especially since as soon as they step out of the elevator, people are shown [[TeleportersAndTransporters Apparating]] and using Floo Powder to arrive at the Ministry. In the book, it's explained that Mr. Weasley thought, since Harry is up on charges of illegally using magic, arriving at the Ministry in as non-magical a way as possible would give a good first impression. [[note]]On a Meta level, it's also [[ChekhovsGun important later]]; the film doesn't explicitly show Harry and the others using it again at the climax, though it could still be the case.[[/note]]

** Mr. Weasley is shown taking Harry to the Ministry of Magic by way of an elevator disguised as a phone booth. This seems to be an odd and impractical way to get there, especially since as soon as they step out of the elevator, people are shown [[TeleportersAndTransporters Apparating]] and using Floo Powder to arrive at the Ministry. In the book, it's explained that Mr. Weasley thought, since Harry is up on charges of illegally using magic, arriving at the Ministry in as non-magical a way as possible would give a good first impression.

The ''Film/HarryPotter'' films are full of these. One major reason why the series is so guilty of this is that the book series was still ongoing while the films were being made, so the scriptwriters had no idea what bits might provide pertinent background for future events. Another reason is that, with the books becoming longer and more intricate, it simply left the films with less and less room to fill in certain background details.

** The shard of the mirror Harry uses to communicate with Aberforth Dumbledore is a slightly odd example -- it was originally given to Harry by Sirius in ''Order of the Phoenix'', in a scene that was left out of that film adaptation, and yet he suddenly has it at the beginning of ''Deathly Hallows Part 1''. ''Part 2'' does explain that Aberforth's mirror was bought from Mundungus Fletcher, who stole it from Sirius's house, as in the book, but there's still no explanation of how Harry's shard came from.

** The film downplays the emphasis on the identity of the Half-Blood Prince. At the end of the film, Snape reveals that he is, in fact, the Half-Blood Prince whose annotated Potions textbook Harry had been learning from on the side. While this does explain the way he's able to save Draco from a spell that Harry would think nobody would know, the film omits the follow-up scene in which Snape asks Harry for his Potions textbook, which clues the readers in beforehand that Snape at least knows about the existence of the Half-Blood Prince's book and instead, the readers are left wondering just why he's so damn ''serious'' about such a thing. While the mystery of the Prince's identity is only mentioned once in the movie, in [[Literature/HarryPotterAndTheHalfBloodPrince the book]] the search for who the Prince really is acts as the main subplot, getting quite nearly as much time as the main plot itself.

** The shard of the mirror Harry broke at the end of the [[Literature/HarryPotterAndTheOrderOfThePhoenix fifth book]] magically appears in the final films and is every bit as useful as it was in the [[Literature/HarryPotterAndTheDeathlyHallows last book]] -- but with no prior explanation.

** Dumbledore's backstory is broadly hinted at -- it's said that his father killed some Muggles, he was once friends with Grindelwald, and he has a brother who isn't very fond of him and a sister who died young -- but it's never explained how any of these events tie together, or how they are relevant to the plot or Dumbledore's character. As well, in the book, Harry grows increasingly [[BrokenPedestal despondent and disillusioned with Dumbledore]] the more he finds out about his mentor's past, but manages to [[RebuiltPedestal rekindle his faith]] in him towards the end. Whereas in the movies, it's never an issue for him, as he explains to Aberforth.
--->'''Harry''': I'm not interested in what happened between you and your brother.

** The movie also never explains how Sirius Black escaped from Azkaban, but leaves in that it's a mystery how he escaped, as no one else ever has. [[Literature/HarryPotterAndThePrisonerOfAzkaban The book]] gives the readers an explanation for how he managed it.

** Another inversion: in the book, Harry casting the Patronus he'd initially attributed to his father was the result of a random EurekaMoment on his part. The film adds in other cases of future characters influencing past events (Hermione throwing the rocks into Hagrud's hut and howling to distract werewolf!Lupin), so that there was a solid basis for his realization there.

** The movie adaptation never bothered to explain that the Marauders were [[spoiler:James Potter, Remus Lupin, Sirius Black, and Peter Pettigrew]]. This makes Lupin's sudden knowledge of exactly what the Marauder's Map does inexplicable, as well as Sirius instantly knowing what the map is when it's brought up later on in the film. Besides, it leaves the map itself an unexplained plot device out of nowhere instead of something perfectly intertwined in the rest of the story. Harry dropping Sirius' nickname in the [[Film/HarryPotterAndTheOrderOfThePhoenix fifth film]] (as well as Pettigrew being called by his) also comes out of nowhere without the Marauder backstory.

** The movie never explains why Hagrid casts spells with a magic ''umbrella'' rather than the standard {{magic wand}}, leaving it as a rather quirky incongruous detail. The book explains that he's a disgraced failed wizard, and he was expelled from Hogwarts as a child. The Ministry destroyed his wand and forbade him to practice magic, but he keeps the fragments of his wand hidden in his umbrella, allowing him to perform a few basic spells with the umbrella when nobody's looking.