Reversing course from the previous game, the Wide-Open Sandbox approach is back and in a big way. The centerpiece of this game is an elaborate recreation of Hogwarts in which you can pretty much go anywhere at any time with nary a loading screen in your way. As such, the gameplay is heavy on running around the castle and performing tasks for various characters, particularly students whom you want to recruit into Dumbledore's Army. However, any part of the original story that took place outside of Hogwarts is skimmed over quickly, including the whole sequence in the Department of Mysteries, which you'd think would have had a lot of gameplay potential.
After Ron and Hermione were Promoted to Playable in the two previous entries, this game reverts to just Harry being the Player Character, with the exception of brief episodes in which you play as the Weasley twins, Sirius Black, and Dumbledore.
Building on having gotten Ralph Fiennes last time, there are now twenty-one of the film actors voicing their characters for the game, including Rupert Grint, Tom Felton, Matthew Lewis, Bonnie Wright, Evanna Lynch, Katie Leung, Luke Youngblood, Harry Melling, Warwick Davis, Natalia Tena, and once again, Ralph Fiennes.
- 11th-Hour Superpower: The final boss against Voldemort has you playing as Dumbledore. His spell casting is even more advanced than Harry's (example: Stupify now fires more than three shots). Sadly, even on Max Discovery level, Harry cannot have this kind of power.
- And Now for Someone Completely Different: This game allows you to briefly control Fred and George, Sirius Black, and even Dumbledore! The DS version has Lupin also playable during the Department of Mysteries battle.
- Big Brother Is Watching: One of the portraits that provides a shortcut is hesitating to give it to Harry, because directly across from him is a portrait with literally a giant watching eyeball looking around. It takes Harry causing a spell to get a curtain to cover the eyeball portrait for him to get the password.
- Don't Explain the Joke: While helping Colin Creevey setting up pranks in the Owlery to be used against the Inquisitorial Squad.Colin:Isn't it funny that O.W.L.s are called owls?! You could take your owl to your O.W.L. How funny would that be?
- Exact Words: In one case involving retrieving a portrait's password (The Man With Faces Three) you first ask him to give you the password. He replies that you'll have to talk to "him" if you want to find the password. The basic explanation for this is... to find the same portrait picture that's on the other side of the shortcut.
- Final Boss: Voldemort, with you playing as Dumbledore.
- Foregone Victory: In the Dumbledore vs. Voldemort duel, Dumbledore is unaffected by any curses which hit him. The duel can only end with Voldemort's defeat.
- Hopeless Boss Fight: When the Room of Requirement is discovered, Harry has to fight the entire Inquisitorial Squad. The fight is unwinnable — you will lose (if not, run out of time) and Harry will be brought to Dumbledore's Office as per the plot.
- Implausible Deniability: When the trio are looking for talking stone gargoyles to help Dean Thomas to get him to the DA meeting they come across one in which they ask him if he talks but it replies no repeatedly.
- It's Up to You: As is so often the case in these games, Ron and Hermione seem to expect Harry to do all the work. This time, Hermione's recurrent excuse is that she and Ron are prefects now and shouldn't really be breaking rules.
- Lame Pun Reaction: This exchange occurs after the trio and Ginny release one of the portable swamps in the Transfiguration Courtyard.Hermione: (celebrating with the rest) Let's hope this makes Umbridge's name mud at the Ministry!
Harry: Yeah, she'll be swamped with complaints.
Ron: (stops being cheerful, and is now serious) Can we go now?
- Lampshade Hanging: Around Hogwarts, branches of Devil's Snare occasionally pop up, which requires you to use the Incendio spell to get them out of your way. In every instance after the first case, Harry wonders aloud who keeps planting these things.
- Megaphone Gag: Helping the Patil twins apply Babbling Beverage on the loudspeakers will make Umbridge talk nonsense and say silly things (e.g. "I eat frogs").
- Mythology Gag: Among the quests to get a password to gain entry for a shortcut through a portrait, one of them requires you to find a story from the story's Daily Prophet. The one that you can eventually find is of an article that explains that Broderick Bode was strangled by a pot of Devil's Snare, which was delivered to his bedside at St. Mungo's. This was actually a plot point in the book that involves the Department of Mysteries, but was omitted from the film.
- One-Time Dungeon: Grimmauld Place is only visited twice. Once for the beginning tutorial and another during Christmas.
- Rewarding Vandalism: Inverted for once, with this game rewarding you for tidying up Hogwarts instead. Oddly, this is the one game that actually justifies the trope, as there's a whole section of the game where you go around vandalizing the school in defiance of Umbridge's takeover. And yes, you can still get points from tidying up Hogwarts while you're wrecking everything for Umbridge.
- Secret Society Group Picture: Discussed trope. When Colin gets asked to join the other DA members in the Room or Requirement, he asks if he should bring his camera there with him. But then he immediately changes his mind, lampshading that he probably should not because it is supposed to be a secret. Coming from Colin, this is a rare form of intelligence.
- Shaped Like Itself: One of the passwords to the shortcut portraits is simply, "This password is absurd."
- Stealth Pun: Among learning the passwords for portraits in order to get hidden shortcuts, this trope comes into play when trying to find out Percival Pratt's password. When Harry approaches a painting of a shepardess and asks her if she knows the password, she replies that he should ask instead ask the portrait of Google Stump. She further explains that he has answers.
- Truer to the Text:
- Fred and George decide to ditch Hogwarts after sprouting a swamp in a corridor. This is in contrast with the film, which showed them simply leaving after letting off a massive display of fireworks.
- Also, at one point when talking to Hermione, S.P.E.W. is offhandedly mentioned. This is a plot element in the books that is not brought up at all in the films.
- In the book, there's a D.A. member named Zacharias Smith who is notable for his negative attitude about Harry. Nick Shirm plays a similar character in the film, but he's never named onscreen, and the credits only call him, "somewhat doubtful boy." For the video game, Shirm voices the same character, and in the game, he's explicitly identified as Zacharias Smith.
- The game takes a third option with regards to Marietta vs. Cho being the traitor to the D.A. In the book, the D.A. was betrayed by Cho's friend Marietta, but the film pulled a Composite Character and had Cho herself do it, albeit under the influence of Veritaserum. In the game, Marietta is mentioned as Cho's friend, so she still exists like in the book, but it still follows the film with a Veritaserum-influenced Cho doing the betraying.
- Video Game Cruelty Potential: You can hex random students in the hallways. Most will just run away, but some fight back. Fred and George even encourage you to practice your curses on "any passing Slytherin", though you can curse kids from other houses as well. You can curse teachers, but they'll just put you in detention.
- You ALL Look Familiar:
- When you go to find the first years who had skipped detention with McGonagall, they're also the same age as the Trio.
- There's a random Ravenclaw with Seamus' face, just with different hair. It is highly creepy when you see them in the same shot.
- "You!" Exclamation: Filch does this when he catches Fred and George transforming a school corridor into a swamp.