Emma Thompson accepted the role of Professor Trelawney to impress her four-year-old daughter, Gaia. Tilda Swinton was originally offered the role but declined.

Other directors reportedly considered for this installment of the Harry Potter series included Callie Khouri and Kenneth Branagh. Branagh had played a major role in the preceding film, Chamber of Secrets.

Dudley Dursley has no lines. He only laughs at the TV and gives two gasps of surprise.

The effects team spent six months creating the dementors. Originally, Alfonso Cuarón wasn't thrilled with the idea of using CGI and wanted to use more traditional techniques such as puppetry. The filmmakers tried a basic technique with a dementor puppet floating in the breeze but weren't satisfied with the way it looked. Puppeteer Basil Twist showed them a technique that involved putting the puppet in water and shooting it in slow motion with the film reversed. The filmmakers liked the way it looked but realized that this would be an impossible task, so in the end they decided to use CGI to create the dementors.

Honeydukes "is floor-to-ceiling psychedelia" and includes Mexican skulls made of sugar. The cast was told that the Honeydukes candy was lacquer-coated, when in fact it wasn't, to prevent candy from disappearing between takes.

Illusionist Paul Kieve served as a consultant. He taught magic to several members of the cast including Daniel Radcliffe and Emma Watson (Harry and Hermione) and worked extensively to create physical magical effects. He is the first illusionist to have worked on any of the series of films. He is also to make a cameo appearance in the film in a scene in the Three Broomsticks pub.

Filming was halted following the vandalization of the train used as the Hogwarts Express.

The set for Honeydukes was previously used as Olivander's Wand Shop in Harry Potter and the Philosopher’s Stone, and Flourish and Blotts in Harry Potter and the Chamber of Secrets.

David Thewlis was originally considered to play Professor Quirrell in Harry Potter and the Philosopher’s Stone.

The film was offered to director Guillermo del Toro but he opted to do Hellboy instead.

Marc Forster was offered the job of directing this film but declined and did Finding Neverland instead.

The tattoos on Sirius Black's body and hands are borrowed from Russian prison gangs. They are markings which identify the person as a man to be feared and respected.

Gary Oldman says he accepted the role of Sirius Black because he "needed the work". He hadn't acted for over a year. His last film was Sin which was filmed in 2002.

Alfonso Cuarón coached Daniel Radcliffe in one scene where the latter had to act awed: "Pretend you're seeing Cameron Diaz in a G-string". It worked.

Executive producer Chris Columbus offered the role of director to Alfonso Cuarón after watching Cuarón's A Little Princess.

Alfonso Cuarón had never read the Harry Potter books or seen the first two movies when he was offered the job of director.

When we see the Marauder's Map for the first time, the name 'Newt Scamander' can be seen. In the Harry Potter universe, he wrote the book "Fantastic Beasts and Where To Find Them", but isn't a teacher at Hogwarts.

Because he would be overseeing this film's post-production work, director Alfonso Cuarón declined the offer to direct Harry Potter and the Goblet of Fire. Mike Newell was then chosen by Warner Brothers.

The German subsidary of Warner Bros. tried to cut down the film to secure a more commercial "Not under 6" rating. They submitted several versions but all were rejected by the German ratings board FSK for this rating. In the end the uncut version was released with a "Not under 12" rating.

Warner Brothers supplied ushers at cinemas with night vision goggles to prevent illegal recording and pirating of the film.

The film contains several commemorations to director Alfonso Cuarón and his Mexican heritage. On the fountain in the courtyard in front of the clock tower, there are several statues of eagles eating snakes. This exact image appears on the Mexican flag. Also, among the many candies offered at Honeydukes are skulls made of sugar, which are a popular treat in Mexico on "El Dia de los Muertos," or the Day of the Dead. And, after Dumbledore says his final lines outside the infirmary, he goes down the stairs humming "La Raspa", the Mexican Hat Dance.

The rating in the Netherlands for this film is "not under 9". This rating was created especially for the film, since it was judged to be too scary for 6 year olds and the next rating, "not under 12", would exclude too much of the target audience.

Aware of his fondness for music, Gary Oldman presented Daniel Radcliffe with a bass guitar as a gift when they met.

In the scene where Harry is given the Marauder's Map by the Weasley twins, the name "Moony" is misspelled as "Mooney". While not really a mistake, there is still an interesting connection. The film's visual effects supervisor is named Karl Mooney. The spelling was changed deliberately for the in-joke.

When Harry, Hermione and Ron are returning to the school from Hagrid's hut after witnessing Buckbeak's execution, Hermione hugs Ron and Harry hugs Hermione, a reference to Alfonso Cuarón's movie Y tu mamá también.

The set for the sequence where Professor Lupin teaches Harry to defend against the dementors previously served as Dumbledore's office in Harry Potter and the Chamber of Secrets.

When the Boggart takes the form of a giant snake, the "snake pit" theme from Raiders of the Lost Ark is worked into the soundtrack by John Williams who also did the music for that film.

Sir Cadogan, played by Paul Whitehouse, was cut almost completely out of the film. You do still see Sir Cadogan jump into the shot of Ron, Harry and Hermione right after Ginny tells them the Fat Lady is gone. He appears in a picture behind them.

A fire near the Glenfinnan viaduct in Scotland delayed filming of the Hogwarts Express scenes for a day.

Two Knight Buses were built: one for exterior shots and one for interior shots. The exterior of the bus was created by taking an ordinary double-decker London bus, adding a level, and painting it purple.

Ian McKellen turned down the role of Dumbledore. Having appeared as Gandalf in The Lord of the Rings, he said, "I had enough trouble living up to one legend. Two would be too much to hope for."

Ron's dream about spiders making him tap-dance is a two-in-one reference of the next book. In Goblet of Fire there is an unforgivable curse where you can control what someone does, taught by Professor Moody. He makes a spider tap-dance on the desk.

Prior to its release, material related to the film was labeled with the code title "Radiator Blues".

Professor Dumbledore seemingly stalls the executioner by saying that he needs to sign the execution order, and that he has "a very long name". In Harry Potter and the Order of the Phoenix, his full name is revealed to be Albus Percival Wulfric Brian Dumbledore.

During the scene where we first meet Buckbeak the Hippogriff, just after Ron pushes Harry forward there is a shot of the animal pooping. The CGI team believes this might be the first example of a CGI animal doing this, but the BBC series Walking with Dinosaurs had done it first.

Alfonso Cuarón had an idea for there to be tiny people inhabiting Hogwarts, and jumping on piano keys in one scene. J.K. Rowling firmly vetoed it, saying tiny people were completely foreign to the world of her books.

When Chris Columbus was still slated to direct, Robson Green was considered for the role of Sirius Black.

Shot entirely on wide-angle lenses. The length of the lenses ranged between 14-24mm and never beyond that. Alfonso Cuarón insisted on the wide-angles because he wanted to let the audience see the foreground and the background.

During the filming of the sleeping bag scene, director Alfonso Cuarón and Alan Rickman played a practical joke on Daniel Radcliffe by hiding a remote-control-operated Whoopee Cushion in his sleeping bag. According to Cuarón, Daniel tried really hard to stay in character while everyone else was laughing.

Two Persian red cats were used for the role of Crookshanks: Crackerjack and Pumpkin.

Much of the filming was done in Scotland, so the filmmakers wanted to be sure that a Highland Cow, a breed of cattle native to Scotland, appeared in the film. The large, hairy animal with big horns can be seen in front of a shop in Hogsmeade when Ron and Hermione go and look for Harry after he leaves the pub.

Among the difficulties associated with filming in Scotland was the amount of rain that fell during the shoot. During the breaks in filming, helicopters delivered large bags of gravel to the set in order to prevent it from washing out. Many of the filmmakers were concerned as to how this would affect the look of the film, although director Alfonso Cuarón and cinematographer Michael Seresin insisted that it was the best look that they could have come up with.

A clause in Alfonso Cuarón's contract forbade the director from cursing in front of the kids on set.

In order to acquaint himself with his three lead actors, director Alfonso Cuarón had each of them write an essay about their characters, from a first-person point of view. Emma Watson, in true Hermione fashion, went a little overboard and wrote a 16-page essay. Daniel Radcliffe wrote a simple one-page summary, and Rupert Grint never even turned his in.

The lyrics to the song the choir sings in the Great Hall, "Double, Double; Toil and Trouble..." and the tag line of the movie "Something wicked this way comes" are made up of lines from William Shakespeare's "Macbeth" (Act 4, Scene 1). The words are recited by the three witches, referred to throughout the play as the Weird Sisters. In Harry Potter and the Goblet of Fire, a band called 'The Weird Sisters' come to play at Hogwarts.

The tune played on the gramophone during the Boggart scene is NOT Louis Prima's "Sing, Sing, Sing". The actual song, "Swing, Swing, Swing" written by John Williams, is from the film 1941.

Cinematographer Emmanuel Lubezki (frequent collaborator of director Alfonso Cuarón) was supposed to be working on this film, but due to scheduling conflicts he was replaced by Michael Seresin.

Director Alfonso Cuarón's wife, Annalisa Bugliani and daughter, Tess Bu Cuarón appear in the film as the mother and baby in the portrait next to the Fat Lady's portrait.

The symbols under Sirius Black's picture on the Wanted Poster translate as "more or less human."

An additional set was built for the candy shop: the cellar. Not used in the film, it was included in a special feature on the DVD release that allowed viewers to explore the shop.

There are at least two moments in the film where sounds from the first film, Harry Potter and the Philosopher’s Stone, are used again. When Harry gets off of Buckbeak a fellow student is heard saying, "That was wicked, Harry". It is the exact recorded sound from the first film where a fellow classmate says the same thing to Harry after he catches Neville's Remembrall whilst on a broom. Also, the sound of the Gryffindors cheering during the Quiddich match is exactly the same as in the first film.