The Minor Harry Potter Character You Likely Forgot Introduced The Room Of Requirement
The "Harry Potter" movies are well-known for skimping on some of the content that made the books so rich and exciting for many fans. Of course, it would be impossible to fit much of that material into the runtime of a blockbuster film (you can expect each of the eight films in the series to last between 2-3 hours, which is still a bit higher than the 90-100 minute average for a film, as calculated by Towards Data Science).
But the franchise's corner-cutting rubs some fans the wrong way when it comes to certain aspects of the Wizarding World, even though it's meant to make the storytelling more efficient without removing too many of the necessary details needed to make the plot comprehensible. This is perhaps why Peeves the poltergeist, a recurring minor character in the books and a fixture of Hogwarts School of Witchcraft and Wizardry, was not included. Other characters also got the shaft (Neville Longbottom's and Nymphadora Tonks' parents, for example) or had their roles greatly reduced, like Tonks herself (Natalia Tena), Remus Lupin (David Thewlis), and pretty much all the house-elves.
If you're in the habit of rewatching the movies but not so much rereading the books, you might have forgotten that the Room of Requirement, which Harry uses to teach his own version of Defense of the Dark Arts, is introduced by someone different than in the films.
The fifth movie uses Neville and Hermione to reveal the room's existence
In the fifth film, as in the book, the regime of Dolores Umbridge (Imelda Staunton) includes a dumbing-down of the Defense Against the Dark Arts classes at a time when Harry (Daniel Radcliffe) and his friends believe those skills are needed most, given Voldemort's (Ralph Fiennes) return. The usually by-the-book Hermione (Emma Watson) thinks they should go behind Umbridge's back and start their own club to learn methods of protecting themselves, with Harry as their teacher. But first, they must figure out where to hold classes in secret. As Harry walks with others interested in the idea, Ginny (Bonnie Wright) suggests the Shrieking Shack, which was in the third movie proven not to actually be haunted, while Hermione thinks the Forbidden Forest might make a good meeting point. Harry decides they'll all come up with a few suggestions for consideration.
In the very next scene, Neville (Matthew Lewis) comes across a new door that appears in the Hogwarts corridors. When he tells the group, Hermione exclaims, "You've done it, Neville. You've found the Room of Requirement." She then explains that it's also known as the "Come and Go Room" and that it only appears when a person has need of it, always equipped for the user. "It's brilliant," Harry says. "It's like Hogwarts wants us to fight back." This exchange implies that the Room of Requirement has appeared at this time and place deliberately.
The Room of Requirement plays an important role in the Harry Potter story
But in the books, the Room of Requirement is a bit more elusive and not quite so purposeful. It is first mentioned in the fourth book of J.K. Rowling's series, "Harry Potter and the Goblet of Fire," when Albus Dumbledore tells Igor Karkaroff that he turned the wrong way going to the bathroom and ended up in a "beautifully proportioned room I have never seen before, containing a really rather magnificent collection of chamber pots" (Chapter 23).
The room features quite prominently in the fifth book ("Harry Potter and the Order of the Phoenix") as the site of the Dumbledore's Army meetings, and it also figures in the sixth and seventh books, similar to its on-screen portrayal. In "Harry Potter and the Half-Blood Prince," Draco Malfoy doesn't fully understand what the Room of Requirement does, but he spends plenty of time there (in the room's guise as the Room of Hidden Things) trying to fulfill his duties for the Dark Lord — including fixing a Vanishing Cabinet that will allow Death Eaters to travel from Borgin and Burke's store in Knockturn Alley to Hogwarts.
In "Harry Potter and the Deathly Hallows – Part 2," Ravenclaw's diadem (and Voldemort's Horcrux) is finally found in the Room of Hidden Things, during a scene in which Vincent Crabbe (Jamie Waylett) uses Fiendfyre to completely destroy the place. It's also where Dumbledore's Army (minus Harry, Hermione, and Ron) hides out as its students get on the wrong side of the new, Dark Arts-friendly regime at Hogwarts. But it's not one of the students who initially discovers the room for the club's use.
In the books, Dobby introduces Harry to the room
In the fifth book, Sirius Black, who has discovered Harry's plans for a secret club, is the one who suggests the Shrieking Shack for a meeting place, rather than Ginny. And the character responsible for bringing the Room of Requirement to Harry's attention doesn't even appear in the "Harry Potter and the Order of the Phoenix" film.
One night, Harry is awakened from a dream by the presence of Dobby the house-elf, who is employed in the Hogwarts kitchens. Dobby, eager to help as always, is returning Harry's pet owl, Hedwig, after she has healed from an attack. Harry asks the house-elf for a place Dumbledore's Army can practice Defense Against the Dark Arts, and Dobby clues him in to the presence of the Room of Requirement, claiming that he learned of it from the other Hogwarts house-elves and has used it to help his fellow house-elf, Winky. Dobby — not Hermione — tells Harry its history, saying, "Mostly people stumbles across it when they needs it, sir, but often they never finds it again, for they do not know that it is always there waiting to be called into service, sir" (Chapter 18). Harry quickly makes the connection to what Dumbledore told Karkaroff in "Goblet of Fire" and asks Dobby exactly how to find it and get in.
Dobby's truncated role in the movies relegated him to only the second and seventh movies, although there is a house-elf cameo in the fourth that looks very like him (via Insider). In the fourth book, though, Dobby was also the original source of Harry's Gillyweed brainstorm, proving that the movies did the house-elf a bit dirty by turning his contributions into Neville's more than once.