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).
(At around fifty-eight minutes) When they discover the Room of Requirement, Ron asks if it could become a bathroom if the user really needed it. This is a reference to the novel "Harry Potter and the Goblet of Fire" in which the Room of Requirement is described for the first time by professor Dumbledore. Although the name of the room is not specified, Dumbledore mentions taking a wrong turn while looking for a lavatory, and finding himself in one he didn't know existed. He was unable to find it again later, making him wonder if the room only appears when the person entering it has an exceptionally full bladder.
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." Unbeknownst 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.
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.