Mike Newell decided against the studio's original idea of adapting the extremely long book into two separate films to be released several months apart, figuring that he could cut enough of the book's bulky subplots to make a workable film. It was Alfonso Cuarón, the director of Harry Potter and the Prisoner of Azkaban, who convinced him.
A digital "spot removing" technique (which had previously been used for such TV shows as Desperate Housewives) was applied in post-production to clear up some of the more severe teen skin problems, since make-up tested poorly for close-up shots in particular.
Rumors on the Internet of a Scottish actor being cast as Viktor Krum were halted when it was announced that, as originally planned by the producers, a Bulgarian would in fact play the part. Perhaps these rumors arose because the Bulgarian was discovered in England and not in Sofia, where there was a casting call for the part of the International Quidditch Champion. Stanislav Ianevski, a Bulgarian student living in London, has been cast in the role.
During the underwater filming Daniel Radcliffe, a couple of the cast members, and all of the underwater crew posed for a photo which he later sent out as a Christmas card with Rudolph noses and antlers Photoshopped on everyone's faces.
Safety divers swam in with scuba regulators to allow them to breathe without having to surface. These scenes were shot in a huge purpose-built tank with a blue-screen background. Daniel Radcliffe alone logged around 41 hours 38 minutes underwater during the course of filming. At one point during training he inadvertently signaled that he was drowning, sending the crew into a huge panic to bring him back up to surface.
Mike Newell originally decided not to have the make-up on Ralph Fiennes to give a more scary Voldemort. But he changed his mind after seeing a minimal make-up design by Nick Dudman. To complement the make-up, Fiennes shaved his head bald as well as his armpits.
The inscriptions on the Riddle's family tombstone had to be digitally changed at the last minute after fans of the books, having seen promotion stills released from the graveyard scene, pointed out that none of the Riddle names on the tombstone referred to Voldemort (a.k.a. Tom Marvolo Riddle), as the filmmakers erroneously thought, but to his dad, Tom Riddle (Sr.), his grandfather, Thomas Riddle, and his grandmother, Mary Riddle, all three of whom were killed by Tom Marvolo Riddle in 1943.
As Ron, Harry and Hermione are talking in the great hall, before George and Fred attempt to enter their names in the Goblet, Hermione is reading a book. We can't see the title, but the cover features the harlequin pattern of the UK editions of the Harry Potter novels.
Mike Newell staged a brawl with one of the twins, both to demonstrate what he wanted for a scene between the twins, and also to undermine his own "authority figure" status ("They were calling me 'sir'!") The fight got so intense that he fractured a rib in the process.
The tune you hear the teens sing to the school song, "Hogwarts, Hogwarts, Hoggy Warty Hogwarts" was written by Mike Newell. The lyrics first appeared in the first novel, "Harry Potter and the Philosopher's Stone". (Traditionally the song can be sung to any tune the singer happens to like.)
The kids had around three weeks of dancing practice for the Yule-ball waltz. Daniel Radcliffe however, appears in almost every scene of the entire film, and thus had only four days to prepare for this task. In several interviews he's given that reason for why his dancing is shown mainly from the waist up (to avoid showing his fumbling feet!).
In the first draft of the script, a subplot featuring the Weasley twins and Ludo Bagman, the head of the Ministry's sports department, was featured prominently. In fact, it was reported that Martin Landham was cast as Bagman. In the subsequent drafts, the subplot was dropped, and the character of Ludo Bagman makes no credited appearance in the movie.
The elves Dobby and Winky were cut due to time constraints. However, if you watch carefully in the first campsite scene, right after Ginny points to something and says "Look!" you can see two House Elves riding on llamas. They go by very fast, so they're hard to see.
The line Moody uses "I know stories about your father that would curl even your greasy hair" is actually a line from the book but in a different scenario: Rita Skeeter says it to Hermione about Ludo Bagman (a character omitted from the movie).
Costume designer Jany Temime considered Hermione's dress for the Yule Ball as the most important, comparing it to that of Cinderella. The design of the dress was changed several times before the designers were satisfied with the results. Emma Watson was very careful not to wear it more than necessary because she was afraid that she would wreck it.
This is the first Harry Potter film to receive a "PG-13" rating or its international equivalent (for sequences of fantasy violence and frightening images). The preceding films having been rated "PG" or one of its international equivalents.
Ralph Fiennes was not wearing any make up to cover his nose. In order to make the character scarier, film editors digitally removed it. Also, the "red, snake-like eyes" the novel describes were not added, due to the actor's thought that the expression in his eyes would provide a better idea of Lord Voldemort's insanity and malignity.
Features one of the largest underwater sets ever constructed. It has the capacity of up to 500,000 liters (132,000 gallons) of water. The largest underwater set constructed was for The Abyss, which had a capacity of 7,000,000 gallons.
The rock band at the Yule Ball is comprised mostly of members of Pulp and Radiohead. In the run-up to the movie, a Canadian folk group called the Wyrd Sisters filed a $40-million lawsuit against Warner Brothers, the North American distributor of the film, Jarvis Cocker from Pulp and Jonny Greenwood and Phil Selway of Radiohead for the use of their group's name. In the book, the band is called the "Weird Sisters" after the witches in William Shakespeare's "Macbeth" but was reportedly renamed the "Wyrd Sisters" for this film. Before the movie was released, however, Warner Brothers removed all references to either name for the band. (In a deleted scene included on the DVD, Professor Flitwick introduces the band as "The band that needs no introduction.") Nevertheless, the Wyrd Sisters moved for an injunction in a Canadian court to prevent distribution of the film in Canada. This motion was dismissed by an Ontario judge.
Early drafts had Ron's estranged brother Percy appearing in a key supporting role but it was written out in the final drafts. In an interview, Chris Rankin, who plays Percy, revealed that his contract of the franchise stipulates that he must appear in four films; the first three, with the option of appearing in either this movie or the next one, Harry Potter and the Order of the Phoenix. Given the fact that Percy appears much longer in the latter, he opted out of the film in favour of appearing in the next one.
First film in the series not to be dubbed into Icelandic (and other languages alike), where dubbing for theatrical release is limited to projects primarily aimed at children. In fact, even with different ratings of the following films, dubbing did not resume for some of those languages (like Icelandic).
First film in the series not to include any scenes from a Quidditch match in Hogwarts. No Quidditch matches were mentioned in the book due to the pitch being used for the maze, this is pointed out soon after the students go back to school.
Icelandic moviegoers (particularly the younger crowd) tended to crack up unexpectedly in theaters when Rita Skeeter first introduces herself. Apparently the audience weren't expecting the pronunciation of her last name, Skeeter, to sound so close to the Icelandic verb "skíta", which happens to be a rather crude word for defecating.
The name of "Durmstrang Institute" appears to be paying homage to the "Sturm und Drang" (translation: Storm and Stress) movement in German literature. The movement, which flourished from around 1770 to 1785, was distinguished by its theme of youthful genius in rebellion against accepted standards, much like Harry and his feelings about the wizarding world's attitude toward the Dark Lord.
In the scene with Dumbledore, Harry and the Pensieve, pay close attention to the glass cabinet Dumbledore approaches while explaining the Pensieve. In the top left corner is a 3D model of what becomes an important symbol in the final novel, Harry Potter and the Deathly Hallows.
After the second task, as Dumbledore magnifies his voice, his wand can be seen with a white band on it that bears the mark of a straight line atop a few lines in a criss cross pattern all atop a circle, which will all be important plot elements in future installments.
The creature Mad-Eye Moody torments in his "dark arts" class (a spider in the book and the original script) is virtually identical to a real animal: the tailless whip-scorpion. Tailless whip-scorpions, of which some 130 species are known, fall in their own taxonomic order 'Amblypygi' under Class 'Arachnida', and are neither scorpions or spiders. Though Moody avers that this one is deadly, real Amblypygids are quite harmless (though almost as big).
This is the first movie to not show the Dursleys. They feature in the book in the very beginning, where they are visited by the Weasley family who come to pick up Harry for the Quidditch World Cup. This was omitted from the movie to save screen time for the main plot.
Mike Newell was not aware that Alan Rickman wore black contact lenses for the role of Snape until one day, when he was complimenting Alan on the amazing shade of his eyes, Alan leaned over and popped one of the lenses out.
In the books, Parvati Patil of Gryffindor has an identical twin Padma Patil of Ravenclaw, to illustrate the unpredictability of the Sorting Hat. In the films; however, Parvati (Shefali Chowdhury) and Padma (Afshan Azad) are not only both in Gryffindor but are played, surprisingly, by unrelated actresses.
In the movie the audience is given the impression that Beauxbatons is an all girls magical academy whereas, Durmstrang is an all boys one. In the books however both schools are co-ed, and in fact in the books the Patil twins leave Harry and Ron to spend time with boys from Beauxbatons.
Director Mike Newell wanted the Dark Mark to "bubble up" out of the skin of Voldemort's followers. The makeup department achieved the effect by applying variations of a silicone skull-and-snake to each actor's arm, gradually making the mark appear more "raised and angry looking". Digital effects were then used to create the snake's movement.
In the extended version of the movie that airs on ABC Family, when Harry is questioned by Snape about stealing from his potion/ingredient closet after Snape says "Boomslang Skin?" "Lacewing Flies?" Snape shuts the door on Harry. The line "You and your little friends are brewing Polyjuice Potion, and believe me, I'm going to find out why." is omitted from this version but appears on the DVD.
The trivia items below may give away important plot points.
There were three characters that had big parts in the book that were completely written out of the film. These characters were: Ludo Bagman, head of the Ministry of Magic's sports department and tournament judge; Winky: the former Crouch house elf that was believed to have cast the dark mark at the Qudditch World Cup; Bertha Jorkins: a woman who was tortured by Voldemort and Wormtail to tell about the Tournament being held at Hogwarts before being killed by them. Frank Bryce walked in Voldemort and Wormtail discussing her death.
The occasional tongue flick done by the character of Barty Crouch Jr. was, in fact, not in the book at all, and was improvised, on the spot, by David Tennant. Brendan Gleeson then added the quirk to a few of his scenes as a hint to the Crouch/Moody substitution. In fact, after the second task, when "Moody" and Crouch senior are talking, Crouch's reaction to seeing Moody do this could suggest that he might suggest something about Moody's true identity.
At the Quidditch World Cup, Amos Diggory says "Parting of the ways?" to Arthur Weasley, and they separate to go to their respective tents. This is a line from the book, only in different context. In the book, Dumbledore says this to Cornelius Fudge, when Fudge refuses to admit that Voldemort has returned. For pacing reasons, this epilogue was deleted from the movie, in favor of using it as a subplot for Harry Potter and the Order of the Phoenix.
Voldemort identifies four of his Death Eaters by name: MacNair, Crabbe, Goyle, and Lucius Malfoy. Lucius Malfoy is the father of Draco Malfoy. The Crabbe and Goyle that he names are the fathers of Draco's friends, who are also usually addressed by their last names. The last one, MacNair, is identified in the book as the would-be executioner of Buckbeak in Harry Potter and the Prisoner of Azkaban.
The writing out of Dobby in this film (Harry Potter and the Order of the Phoenix) resulted in a slightly larger role for Neville Longbottom. In this film, it is Neville, rather than Dobby, who gives Harry the gillyweed for the second task, and we later learn that this idea was given to him by Barty Crouch Jr./Professor Moody. In the book, Crouch/Moody explains that his first approach was in fact to give Neville the book where gillyweed could be found, but Harry didn't ask Neville because he wasn't supposed to ask for help so he only told Ron and Hermione what he had to do. So Moody arranged for Dobby to overhear him talking about it. This removal creates a rather uncharacteristic byproduct - Neville, who is supposed to be terrified of Professor Snape, would have had to steal the Gillyweed from Snape's store.