The D.A. or Dumbledore's Army forms in Harry Potter and the Order of the Phoenix. It becomes a necessary training course due to Dolores Umbridge and the Ministry of Magic's refusal to allow students to practice spells. The D.A. is consistent with mainly Harry's closest group of friends along with a variety of students from Gryffindor, Hufflepuff, and Ravenclaw houses.
The fifth book is chock-full of anecdotes about this motley crew of rebellious teens, but as per usual, screen time just could not permit showing all of the delicious details in its film adaptation. Because book readers are familiar with all the backstories and explanations about this group, hopefully, this list serves to fill in some of the inevitable questions you picked up after the film. Here are ten things about Dumbledore's Army that only make sense if you read the books.
10 The Name
The name Dumbledore's Army is actually suggested by Ginny Weasley as a way to add insult to injury to the Ministry of Magic. During their takeover of Hogwarts, the Minister of Magic allows Umbridge to act as Hogwarts High Inquisitor in order to prove their authority over Dumbledore. Cornelius Fudge fought to sway Dumbledore and Harry's claims of Voldemort's return all summer and took no shame in dragging both of their names through the mud. For some unexplained reasoning, Fudge convinces himself that Dumbledore is out for his job and thinks that Dumbledore is recruiting an army against the Ministry.
The name is meant to be a private joke in reference to this absurd claim, but it causes trouble later on when Fudge is shown the list that says Dumbledore's Army.
9 Meeting In The Hog's Head
The initial meetings for the D.A. are held at the Hog's Head, a local bar inside of Hogsmeade. Hermione chooses this ambiguous location thinking that no one would of importance would be in there to overhear their strategies. The film quickly runs over this scene, which gives the illusion that the students only show up to hear from Harry first-hand about Cedric Diggory's death. When Harry openly admits to thinking he might not be the best teacher, his peers encourage him by speaking to his early accomplishments. In the book, this is also when Harry tells his classmates about his parent's murder and everything else he has done thus far to fight Voldemort.
It's also revealed later on the Mundungus Fletcher is in the bar disguised as an old witch dressed in black. It's him who relays the inner workings of the D.A. to the Order of the Phoenix. The bar's owner, Aberforth Dumbledore, also secretly fills his brother in on this meeting.
8 The Room of Requirement
The Room of Requirement is eventually utilized to hold meetings for the D.A. The Room of Requirement (also known as the Come and Go Room) is actually shown to Harry by Dobby the house-elf, although it is Neville who stumbles upon the room in the film.
This magical room is somewhat sentient as it appears only when someone is in need of it. It can appear as any type of room. For example, it once turned itself into a restroom when Dumbledore was in desperate need of one.
7 How They Knew When To Meet
Hermione thoughtfully founded Dumbledore's Army and, naturally, became in charge of coordinating their meetings. They didn't want any of the students (particularly the Inquisitorial Squad) to know of their ventures, so Hermione devised a communication device.
Every D.A. member was given a coin bewitched with the Protean Charm. This is a high-level charm (conjured by Hermione, of course) that connects multiple objects for a central purpose. Whenever Harry activated the coins, it would let the D.A. members know the time and date of their next meeting.
6 The Sign-Up Sheet
The signup sheet actually holds a more sinister purpose than the film lets on. Audiences see as eager students line up in the Hog's Head to sign in their names for the cause. In the books, Hermione secretly bewitches this bit parchment with the member's names on it, which would enact a jinx if provoked.
The purpose of this curse is to identify the culprit should anyone betray the D.A. In the film, Cho Chang turns the group into Dolores Umbridge, but obviously, nothing really happens to her that would give way to Hermione's curse. Which is simply because the cursed parchment didn't exist in the movies.
5 Why Cho Chang Turned Them In
Umbridge casually explains that Cho Chang gives in to her questioning about Harry's activities because of the truth serum, Veritiserum. Cho Chang leads them right to their hideout because, even under the potion, the room is something the Cho Chang is in desperate need of.
However, in the book, it is Cho Chang's friend, Marietta Edgecombe, who willing told Umbridge of the group's secret hideout. Marietta had not been entirely convinced of Harry's stories about Voldemort. And when Umbridge enlisted in the help of Marietta's mother, who worked at the Floo Network, Marietta feared her mother's job might be jeopardy. She went straight to Umbridge. This is when Hermione's jinxed parchment comes into play as Marietta's face immediately becomes covered in boils to spell out the work, "SNEAK."
4 Fred and George's Departure
Fred and George's departure scene is, arguably, one of the greatest scenes in the books. It's vastly underplayed in the films and seemingly comes out of nowhere. What audiences can assume is that, like the rest of Hogwarts, they become fed up with Umbridge's strict rule (especially after Dumbledore's leave) and decide to leave with a literal bang.
In the books, their departure is a bit more elaborate. The two cause chaos and destruction in their wake, leaving swamps, Dungbombs, and all sorts of joke-related nuisances throughout the castle. Soon, other students (and even Peeves) follows suit, causing the Hogwarts castle to crumble under Umbridge's rule. This seems to happen in just the short span of their departure in the film, but this terror actually goes on for weeks.
Vertaserium is a potion that forces the drinker to reveal their deepest secrets. Umbridge threatens to use this on Harry when he and the key members of Dumbledore's Army are brought into her office by Draco and the rest of the Inquisitorial Squad. But just exactly had this group been doing before getting caught?
Harry and Hermione were on their way to Umbridge's office in one last attempt to get ahold of Sirius using the only private Floo system left at Hogwarts: Umbridge's fireplace. The rest of the D.A. set up distractions around the school, which didn't seem unusual thanks to Fred and George. However, Umbridge saw through their ruse and had Luna, Neville, Ginny, and Ron reprimanded and brought to her office. This is when she (mistakenly) believes that Snape will help her use Veriteserium to interrogate Harry and his friends.
2 Ginny's Spell In The Department of Mysteries
In one scene in the films, Ginny uses a powerful spell in the Department of Mysteries that blasts the Death Eaters back. This is meant to be a characterizing moment for Ginny but probably went unnoticed by people who have not read the books.
Ginny is showcased as a talented and very powerful witch in the books. This scene is meant to shed some light on this aspect of Ginny, but the result of that scene is less than spectacular. Ginny displays her superior magic abilities particularly during her practice sessions in Dumbledore's Army. Which makes even more sense as to why Harry falls for her, even if the movies don't show it.
1 Neville's Leadership In 'Deathly Hallows'
The D.A. disbands after Dumbledore returns to Hogwarts by the end of Harry's fifth year as there is no longer a need for private Defense Against the Dark Arts classes. However, the need for the D.A. rises again (unbeknownst to Harry, Ron, and Hermione) in the Deathly Hallows.
Neville, of all people, actually takes the reins and reestablishes Dumbledore's Army in order to rebel against the Death Eaters who have taken over Hogwarts. It causes Neville to be subject to harsh discipline, but he strives forward, eventually leading the other members to retreat permanently in the Room of Requirement. What started off as a silly name that poked fun at the Ministry, actually came to be a full-fledged army under the leadership of Neville Longbottom during the 1997-1998 school year.