Director Guillermo del Toro turned down the chance to direct this film so he could work on Hellboy II: The Golden Army.

Anand Tucker and Michael Hoffman were both rumoured as possible directors for this film.

Helen McCrory had been cast to play Bellatrix Lestrange in Harry Potter and the Order of the Phoenix, but had to back out because she was pregnant. She will play Bellatrix's sister, Narcissa Malfoy in this film.

Jessie Cave beat over 7000 girls to win the role of Lavender Brown, Ron's "love-interest" from the book.

Hero Fiennes-Tiffin has been cast as Tom Riddle, Age 11, while his uncle, Ralph Fiennes plays Lord Voldemort (formerly Tom Riddle). His parents are Martha Fiennes (Ralph's sister) and George Tiffin.

Quidditch at Hogwarts makes a grand (and much publicized) return, after being completely absent since the fourth instalment, and barely present in the third. A full game of Quidditch has in fact not been featured since the 2nd film.

Robert Knox, who plays Marcus Belby, was tragically stabbed to death on May 24, 2008, just days after filming wrapped.

Director David Yates says that he hired Hero Fiennes-Tiffin to play the young Voldemort/Tom Riddle because of his resemblance to his uncle Ralph Fiennes (who plays Voldemort/Tom Riddle as an adult) but not specifically because he was the actor's nephew. He liked the dark haunted quality about the young actor.

Christian Coulson, who played the 16-year-old Tom Marvolo Riddle in Harry Potter and the Chamber of Secrets, was long rumoured to be reprising his role for this film, but, at the age of 30, it was decided that he was too old for the part.

Naomi Watts was the first choice to play the part of Narcissa Malfoy but dropped out due to scheduling conflicts.

Thomas James Longley auditioned for the role of Tom Riddle, but lost out to Frank Dillane.

Dame Maggie Smith completed filming this film whilst undergoing radio-therapy as treatment for breast cancer.

The original script included all of Dumbledore's memories about Voldemort as outlined in the source novel, but the director insisted on trimming them down as, according to Steve Kloves, "he wanted to showcase Voldemort's rise without getting overly involved with his past as Riddle."

The first Harry Potter film to be rated PG by the MPAA since Harry Potter and the Prisoner of Azkaban.

There is a scene in this movie in which Death Eaters, led by Bellatrix Lestrange and Fenrir Greyback, attack The Burrow where Harry, The Weasley's, Lupin, and Tonks are staying. This particular scene was not in the book, but was made just for the movie to serve as a representative of all the news reports, which are scattered around in the source novel, about various attacks by Death Eaters on the wizard community. It was considered to provide better pacing for a movie to have Harry actually experience one such attack first hand, rather than hearing/reading about those that kept happening to some other students, or their relatives.

J.K. Rowling read through the script for this film, and found a line where Dumbledore mentions a girl he had a crush on when he was younger. After reading it, she informed the filmmakers that Dumbledore is in fact gay, and that his only romantic infatuation was with the wizard Grindelwald, whom he later had to defeat in a wizard duel. She later made this information public while promoting the final book, Harry Potter and the Deathly Hallows.

Two new scenes were added that do not appear in the book: The collapsing Millenium bridge and the Death Eater attack at the Burrow. An attack on a Muggle bridge is actually mentioned in the original opening of the book by Cornelius Fudge, but not shown.

Originally, the shooting script was written so that Harry takes possession of Dumbledore's wand after he is killed. Shortly before filming began, the final book in the series came out, in which Dumbledore's wand, and who possesses it, turn out to be major issues, so the script had to be changed.