Dario Marianelli was considered as a possible composer for this film.

Evanna Lynch beat 15000 girls and 29 finalists for the role of Luna. When it came to viewing the finalists, she was the ninth person in the audition videos and one of the producers, David Barron stopped viewing after her saying, "She is Luna." Unknown to them, Lynch actually wrote a letter directly to J.K. Rowling.

The Department of Mysteries set marks the first time that a totally computer-generated set was used throughout the movie. Building the set practically is too expensive since an estimation of 15000 crystal balls are needed and it will take a lot of time to clean and setup again if a retake is required.

Imelda Staunton was the producer’s only choice for the role of Umbridge. She and costume designer Jany Temime came up with the idea of making her dress more padded and more saturated as the film progresses. The novel describes her being physically chubby and like a toad.

The Room of Requirement was described as a room with no beginning nor end. To achieve the sense of infinity, the effects crew spent 5 months designing the room by having rotatable mirrors installed that would minimize camera and crew reflection as well as to avoid the Hall of Mirrors effect (a common term in CG). Also, another problem on the set is that the lighting underneath the grille was quite bright to generate reflection; so the floor had to be in black, cast members had to have black velvet covering their shoe soles while the crew has to wear blue surgical shoes to prevent treading dust onto the floor set.

Professor Umbridge, though she teaches in a classroom that has appeared in films two through four, inhabits an office vastly different from those of her predecessors. The set was redressed with "fluffy, pink filigree," including a carpet that cost £50,000 to make, and a number of plates upon which moving kittens were animated in post-production. A 24-hour photo shoot was held to photograph and film the kittens for use on these plates. Even the elegant quill which Umbridge gives Harry to write lines using his own blood was designed by the set designers.

Helen McCrory was originally cast as Bellatrix Lestrange, but due to pregnancy she had to be replaced by Helena Bonham Carter. McCrory was later cast as Narcissa Malfoy, who is Bellatrix's sister, in Harry Potter and the Half-Blood Prince.

The radish earrings worn by Luna Lovegood were actually made by Evanna Lynch.

Mira Nair and Jean-Pierre Jeunet were approached to direct the film. Both declined.

The character of Grawp was done by using a recent breakthrough in film technology, developed by Image Metrics, which allows a computer to map an actor's performance onto any character virtual or human, living or dead. The result is said to be far superior to standard hand-drawn computer graphics which are very costly and time-consuming.

During the breakfast scene in the Great Hall, a box of cereal can be seen with the name Cheeri-Owls with a colour scheme similar to that of a Cheerios box.

According to the original script, the character of Kreacher was not intended to be in this film at all. But after J.K Rowling read the script, she more or less insisted on him needing to be there to avoid some serious problems with future installments of the series. So, even if Kreacher still has no noticeable impact on the plot or story as it's presented in the film, a couple of scenes with him were added at the last minute based on this request.

Daniel Radcliffe came up with the idea that, as a gesture of respect to a teacher that Harry most certainly looked up to, he would wear a certain type of clothes that resemble the outfit worn by Prof. Lupin in his lessons from Harry Potter and the Prisoner of Azkaban, during his teachings. The director liked the suggestion, so that became the basis for his look during those scenes.

In Harry Potter and the Chamber of Secrets, when Ron is rescuing Harry from the Dursleys', Ron tells Harry that they were "rescuing you of course". In this film, Moody says the same thing when he rescues Harry.

Harry's, Ron's and Hermione's signatures for the Dumbledore's Army roster are the same signatures seen in the books.

The most expensive set was the 200 foot-long hallway in the Ministry of Magic.

In the scene where Harry, Ron, and Hermione are discussing Harry's kiss with Cho, the three begin to crack up near the end of the scene. This was all real laughter from the three actors. The director thought it was good for the scene and kept rolling.

Many Harry Potter fans went to see Happy Feet just to see a trailer for this film.

Padfoot, Sirius Black's canine alter-ego, was played by a Scottish Deerhound named Cleod.

The portrait in Dumbledore's office that he addresses as "Phineas" is in fact Phineas Nigellus Black, great-grandfather of Sirius Black and, by extension, Bellatrix Lestrange and Narcissa Malfoy; as well as being great-great-grandfather to Draco Malfoy and Nymphadora Tonks.

Much of the Atrium set design was based on the designs of London Underground.

Since this film would include wand dueling at an elite level, a specific "wand choreographer" was brought in to design the style and technique of this highly unorthodox way of fighting. The result consists of five basic spell-casting moves, which each of the actors were then allowed to adapt slightly to fit their own character. So, for instance, Lucius Malfoy would have a very formal and somewhat 'stiff' fighting style, while Sirius Black uses a snappy and more spontaneous 'street fight' style.

The set used for Igor Karkaroff's trial scene in Harry Potter and the Goblet of Fire was doubled in size for Harry's trial in this film, while still protecting its symmetry.

J. K. Rowling provided over 70 names for the Black family tree tapestry, complete with details of relations between each and every member, whose were to be scorched and so on.

In the Atrium of the Ministry of Magic, many offices are seen layered upon each other. In reality, there are only two layers (floors), but at different times, different people were filmed doing different things in the offices and later with the help of computer animation, put together to get the illusion of several layers of office.

Kenneth Branagh was originally set to return as Prof. Gildroy Lockheart in a brief cameo. Originally, Harry was to encounter his former professor in an insane asylum while visiting Ron's dad Arthur Weasley at St. Mungo's. The scene was meant to establish Lockheart as irrevocably scarred from his backfired curse in Harry Potter and the Chamber of Secrets, as well as the insanity of Neville's parents after being tortured by Bellatrix Lestrange. The scene was cut for pacing and budget issues, as it would have necessitated building a new set.

Jean-Pierre Jeunet was offered the chance to direct the movie, but he declined, feeling that he wouldn't have enough creative influence.

The walls of Professor Umbridge's office are covered with kitten plates. In the image on one of the plates, the wall behind the kitten has a kitten plate on it.

The studio considered moving the entire production out to Barradov in Prague from Leavesden to take advantage of its incentives but the move was vetoed by Alan Radcliffe and David Heyman, primarily on security issues.

When Sirius joins Harry in the room with the Black family tree, to the lower left of the door is a Starbucks logo. The "siren" (a mermaid with two tails) has been stylized to be in keeping with the decor of the room, but she is wearing a crown and holding both her fins aloft like the Starbucks logo.

The exchange "So how do we get to London, then? - "We fly, of course!" pays tributes to a very similar exchange in Disney's Peter Pan, "So how do we get to Neverland? - "We fly, of course!"

The only jinx we see Neville Longbottom use successfully against an enemy (Petrificus Totalus), just happens to be the very same jinx Hermione used on him in Harry Potter and the Sorcerer's Stone.

Though the Mirror of Erised doesn't appear on the novel it turns up in the movie. It can be seen during Harry's possession by Voldemort and when Harry draws closer to the mirror his face is shown distorted with some of the Dark Lord's features. David Yates and Stuart Craig decided to use it as there are one more example of the many appeals to objects and shots from previous movies on the series.

Dumbledore's line "Don't fight him, Harry, you can't win" was featured prominently in just about every trailer and TV-spot, yet it is nowhere in the final version of the film, nor in the DVD's extended scenes.

Daniel Radcliffe can't tolerate contact lens, so in the scene where Harry is possessed, his eyes are digitally changed.

Originally the death of Sirius Black appeared just as it does in the novel, with Bellatrix pushing him through the death archway after a longer one-on-one duel. Though filmed, the sequence was cut due to time and pacing issues. A portion of this original death scene remains in the final film: when hit with the killing curse, Sirius inexplicably floats backward though the arch.

Harry's scream at the Department of Mysteries was cut out because it was too agonizing.

Sirius Black's death was foreshadowed and mentioned by Sybil Trelawney in the third novel Prisoner of Azkaban. J.K. Rowling's text line mentioned: "When thirteen dine together, the first to rise will be the first to die." In the dining room in Grimmauld Place, we see thirteen people including Sirius dining together and during a commotion with Mrs. Weasley, he got up first. When Sirius fell at the Department of Mysteries, this means that what Trelawney mentioned was accurate.