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.
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.
During filming of The Order of the Phoenix, Alan Rickman banned Matthew Lewis and Rupert Grint from being within 5 meters of his new BMW, because during the making of The Goblet of Fire, they spilled milkshake in his car.
Since this film would include wand dueling at an elite level, a specific "wand choreographer" (Paul Harris, who is actually a professional dance 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.
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 (2004), during his teachings. The director liked the suggestion, so that became the basis for his look during those scenes.
Evanna Lynch beat 15000 girls for the role of Luna. She was ninth in a line of 29 finalists, and when it came to viewing the audition videos, one of the producers, David Barron, stopped viewing after her audition, saying, "She is Luna." Unknown to them, Lynch actually wrote a letter directly to J.K. Rowling.
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.
During the breakfast scene in the Great Hall, boxes of cereal can be seen with the names Cheeri-Owls and Pixie-Puffs, with color schemes similar to those of Cheerios and Sugar Puffs boxes, respectively.
When Dumbledore's Army first meets at the Hog's Head Inn, there is a bartender accompanied by a goat, played by Jim McManus, and credited as "Barman." This character is later identified as Aberforth Dumbledore, the Professor's younger brother. This character returns in Harry Potter and the Deathly Hallows: Part 2 (2011), and is played by Ciarán Hinds.
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.
When they discover the Room of Requirement, Ron asks if it could become a bathroom if the user really needed it. This is a reference to the novel 'Harry Potter and the Goblet of Fire'. In it, the Room of Requirement is mentioned for the first time, having become a lavatory for Dumbledore.
Several members of the Black family are named after astronomical objects. You can see some of the names in the scene when Sirius and Harry are in the family tree room. Sirius, Regulus, Andromeda, Arcturus, Bellatrix, and Cygnus are the ones that can be seen.
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. Though Kreacher 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.
Although based on the longest book in the seven book series, this is the second shortest movie in the Harry Potter film series. The longest film, Harry Potter and the Chamber of Secrets (2002), is based on the second shortest book.
J.K. Rowling had been a schoolteacher before writing the Potter books, and this film features some references to the British educational system. In the late 1980s and early 1990s, students at age 16 were given Ordinary Level (O-Level) exams in all their major subjects, for which they can receive their General Certificate of Education. This corresponds to the Ordinary Wizarding Level (O.W.L.) exams given at Hogwarts. Students who planned to go on to university stayed on two more years to take their Advanced (A-Level) Exams, which roughly correspond to N.E.W.T. (Nastily Exhausting Wizarding Test) Exams.
Katie Leung was nervous about kissing Daniel Radcliffe onscreen, and spent several weeks preparing herself for it. On the planned day for the shooting of the scene, Radcliffe came down with an illness, and filming had to be postponed while he recovered.
This film reveals several characters to be related to Sirius Black, most notably his cousin Bellatrix Lestrange, whose sister (Narcissa) is Draco Malfoy's mother. One connection that is not made in this film is that Nymphadora Tonks is family as well (first cousin, once removed), as her mother (Andromeda) is sister to both Bellatrix Lestrange and Narcissa Malfoy. Andromeda's name can be seen on the tapestry in Sirious' house, but her portrait has been burned off; the book explains that her family disowned her for marrying a Muggle-born wizard.
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.
In Harry Potter and the Chamber of Secrets (2002), 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.
During filming of the stand-off scene in the Ministry of Magic, Matthew Lewis suffered a perforated eardrum after accidentally moving his head while Helena Bonham Carter had her wand in his ear "..as a sort of Q-Tip...[to] sort of torture [his ear]" Carter said. Lewis was deaf in that ear for a few days afterward, though he laughed about the incident in interviews.
Since Alastor Moody (Brendan Gleeson) has a prosthetic leg, he could not balance properly on broomstick, being unable to use the stirrups. Instead, his broom has posts at the front where he rests his legs, a seat which allows him to lean backwards, and a control stick for his hands. The arrangement is very similar to automobiles made for double-amputees, which have hand controls instead of pedals.
The Department of Mysteries was the first completely computer-generated set used in the series. Building the set practically was too expensive since an estimated 15000 crystal balls would have been needed, and it would have taken a lot of time to clean and set them up again between takes.
Kenneth Branagh was originally set to return as Prof. Gilderoy Lockhart 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 Lockhart as irrevocably scarred from his backfired curse in Harry Potter and the Chamber of Secrets (2002), 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.
Over 30,000 individually placed tiles were used to create the Ministry of Magic sets. As real ceramic tiles would have been far too expensive to produce, they were instead made out of heavily-painted cardboard.
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.
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.
The Room of Requirement was described as a room with no beginning or 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, 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 had to wear surgical shoes to prevent treading dust onto the floor set.
Rose Windsor, a member of the British Royal Family, worked - and is credited as - an Art Department Assistant on this film. Her wedding to a member of the Royal Family was featured in OK! magazine - largely due to Prince William Windsor's girlfriend Catherine Duchess of Cambridge attending her wedding. She specifically asked to be credited on Harry Potter as 'Rose Windsor' rather than her full Royal title.
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.
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.
Edgar Bones, the Wizard standing to the left of Dumbledore and directly behind James Potter and Sirius Black, was played by Cliff Lanning (wearing a false mustache), the 1st Assistant Director of the movie.
The trivia items below may give away important plot points.
In the original outline for the novel, the character of Arthur Weasley was to be killed off by Voldemort to entice Harry to the Department of Mysteries. J.K. Rowling couldn't go through with killing off such a beloved character, so Weasley was allowed to survive, albeit gravely injured.
Many fans were critical of the scene where Harry sees Voldemort at the train station dressed in a Muggle suit, saying it was out of character for Voldemort to do such a thing. Director David Yates and producer David Heyman both defended the scene, though each had different takes on it. Yates explained that it was Voldemort's way of taunting Harry, that Voldemort could appear in plain sight in a crowd of Muggles who would not realize how dangerous he is. Heyman, on the other hand, said that it's a figment of Harry's imagination, symbolizing Voldemort taking control of Harry's mind.
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 (2001).
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.
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 as one more example of the many appeals to objects and shots from previous movies on the series.