Helen McCrory was originally cast as Bellatrix Lestrange, but due to her pregnancy, she had to be replaced by Helena Bonham Carter (according to her, the insurance wouldn't cover "pregnant witches"). McCrory was later invited back and cast as Narcissa Malfoy, who is Bellatrix's sister, in Harry Potter und der Halbblutprinz (2009).
Evanna Lynch beat 15,000 girls for the role of Luna Lovegood. She was ninth in a line of thirty finalists, and when it came to viewing the audition videos, producer David Barron stopped viewing after her audition, saying, "She is Luna." Unbeknownst to them, Lynch had written a letter directly to J.K. Rowling.
During filming, Alan Rickman banned Matthew Lewis and Rupert Grint from being within five meters of his new BMW, because during the making of Harry Potter und der Feuerkelch (2005), they spilled a milkshake in his car.
Stephen King said the character of Dolores Umbridge was the "greatest make-believe villain to come along since Hannibal Lecter."
Emma Watson was seriously considering not returning for the other movies in the franchise after this installment, but decided to stay after contemplating that it would be uncomfortable to watch them being made with someone else as Hermione. In Harry Potter 20th Anniversary: Return to Hogwarts (2022), she explained that the problems she had with the role were that she often felt lonely on set, as Hermione was a typical outsider, not only in her class but also within her group of closest friends, where she was the only girl. Watson also had difficulty coping with the instant fame and attention from fans.