As in her previous films, Evanna Lynch designed a lot for her Luna Lovegood character, including what she wore, jewelry and things for the Lovegood's home. She even came up with their dance moves for the wedding scene.
Jason Isaacs originally considered not returning for this film, fearing that his character's arrest and imprisonment at the end of Harry Potter and the Order of the Phoenix (2007) would mean very little if any screen time in the finale. Upon meeting J.K. Rowling, he begged to be let out of prison. She told him "You're out. Chapter one." This immediately convinced him to sign on for the film.
According to David Heyman, the work print of "Harry Potter and the Deathly Hallows" was at 5.5 hours long, and the shooting script was close to 500 pages, which justified the decision to split the movie into two.
In the opening of the film, when Hermione is called down to tea, we can hear her parents talking about Australia along with an announcer, presumably on the TV, taking about a country-side. This is a reference to the book, in which Hermione says she not only wiped herself from her parents memories, but gave them new identities and made them think that they'd love nothing more than to move to Australia, which we presume that they do.
Warwick Davis worked a third job; aside from the two characters he plays in the series, Davis runs a company called Willow (1988) Personnel Management. This company helps little people to find work on film, and found the on-set stand-ins for Dobby and Kreacher.
David Holmes, 25, Daniel Radcliffe's stunt-double, was seriously injured on the set at Leavesden Studios, near Watford, Hertfordshire. He was performing an aerial sequence when he fell to the ground following an explosion, which was part of the stunt, and sustained a serious back injury.
Scabior's violent twist in the air when Bellatrix uses her whip on him was not in the original script. Nick Moran improvised it on the set to avoid being upstaged by Helena Bonham Carter. The filmmakers liked that touch and decided to use it. Moran was delighted, until he realized that, for it to work, that stunt would have to be repeated for several takes.
the scenes featuring Dobby and Kreecher were filmed twice. First they were played physically on set by their respective voice actors, so the other actors and animators had a guideline to work with. Then the same scene was shot without the voice actors, so the SFX team could put in their CG characters in post-production.
At first, this was meant to be only one film, but due to the size of the book, and the decision that nothing could be left out to squeeze into one movie, the producers decided to split it into this film and Harry Potter and the Deathly Hallows: Part 2 (2011).
Frances de la Tour reprises the role of Madame Olympe Maxime (from Harry Potter and the Goblet of Fire (2005)) in this film; the character does not appear in the book. She may be a substitute for Viktor Krum (also from the Goblet story), who appeared prominently at the same scene in the book.
John Williams, who composed the scores to the first three films, expressed his interest in returning to score 'Harry Potter and the Deathly Hallows', but was unable to do so due to scheduling conflicts.
Nick Moran was interested in knowing how his character would appear in costume. He saw earlier renderings and was unsatisfied with some of it. He requested several items to be added, including knee-high boots that had to be laced up 'all the way and were uncomfortable while running. He had no clue he would have to be running in the forest after Daniel Radcliffe, Rupert Grint, and Emma Watson from a good amount of takes.
Stanislav Ianevski did appear in his role as Viktor Krum for the wedding scene, but his scenes were cut from the final film (although promo pictures of him dancing with Emma Watson exist, as well as behind-the-scenes footage).
The character Fenrir Greyback (Dave Legeno) is a reference to Fenris the Wolf in the Nordic mythology. In that mythology, Fenrir bit the arm of Tyr and was involved in Odin's death, triggering Ragnarok. The movie shares some similarities; in the Seven Harry's scene, Bill Weasley (Domhnall Gleeson) remarks how he got his face scarred by Fenrir in a skirmish, just as how Tyr's arm got bitten (note: this was originally mentioned at the end of Half-Blood Prince novel); A small flashback of Dumbledore's murder can be seen at the beginning - in the previous movie, Harry Potter and the Half-Blood Prince (2009), Fenrir was among the Death Eaters involved, similar to Odin's death.
Cast members John Hurt and Bill Nighy have both played prominent roles in adaptations of another well-known fantasy series, The Lord of the Rings. Hurt was the voice of Aragorn in Ralph Bakshi's 1978 film The Lord of the Rings (1978). Nighy was the voice of Sam Gamgee in the BBC Radio broadcast.
Bruno Delbonnel declined to return for the final two films, saying that "I think I was scared of repeating myself." Subsequently, the filmmakers hired fellow French-Portuguese cinematographer Eduardo Serra.
The trivia items below may give away important plot points.
Filming the torture scene where Bellatrix is torturing Hermione at the Malfoy's Manor at the climax proved to be very intense for the actors involved (Most of the scene was cut to avoid an R rating in the USA and a 15 rating in the UK). So intense and brutal as it was that Helena Bonham Carter approached Emma Watson right afterward to make sure they were still on good terms.
Having Bellatrix carve "mudblood" into Hermione's arm during the torture scene was not in the original script, but it was an idea that both Emma Watson and Helena Bonham Carter came up with together on the spur of the moment during filming.
It had been reported that the film would contain a few scenes of Daniel Radcliffe in the nude, leading to speculation among fans that the film would earn a more strict rating. This turned out not to be the case, although the scene in which visions of Harry and Hermione kissing appear to try to deter Ron give the illusion that Radcliffe and Emma Watson are both nude. In reality both actors wore jeans and Watson was provided with a strapless brassiere so she would not have to be completely topless. Director David Yates said that complete nudity was not necessary as the characters would be partially obscured by fog.
The revelation that R.A.B. was Sirius Black's brother Regulus was actually correctly guessed by book readers soon after 'Harry Potter and the Half-Blood Prince' was published, and a good two years before 'Deathly Hallows' was. Of all the surnames that J.K. Rowling came up with for the series, Black is the only one that readily translates into other languages. In foreign language editions of the book in which this surname is translated, RAB was similarly altered, such that the B always matched the first letter of the word for black. For example, Dutch editions translated Sirius Black as Sirius Zwarts, and R.A.B. to R.A.Z. Finnish editions used Sirius Musta and R.A.M.
Madam Hooch (Zoë Wanamaker) and Gildroy Lockheart (Kenneth Branagh) are the only two Hogwarts faculty members to survive all the previous installments of the Harry Potter series and not appear in this film. Though not addressed in the films, Lockheart suffered brain damage from a backfired spell and was confined to a wizard insane asylum, and Hooch was written out of the series after Wanamaker declined to return to the role after the first film.