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 and the Half-Blood Prince (2009).
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 and the Goblet of Fire (2005), they spilled a milkshake in his car.
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." Unknown to them, Lynch had written a letter directly to J.K. Rowling.
Imelda Staunton was the producers' only choice for the role of Professor Dolores Umbridge. She and costume designer Jany Temime came up with the idea of making her dress more padded and more saturated as the movie progresses. The novel describes her being physically chubby, and like a toad.
J.K. Rowling provided more than seventy names for the Black family tree tapestry, complete with details of relations between each member.
Timothy Spall: (At around twenty-six minutes) Reprising his role of Peter Pettigrew (Wormtail), seen only in the moving photograph of the original Order.
David Yates: Near the end of the movie, he appears in a portrait in the room with the moving stairs.