The barman seen with the goat at the shabby Hog's Head Inn where Harry and Hermione hold the first Dumbledore's Army meeting is Aberforth Dumbledore, Albus' brother. In the novels he runs the Hog's Head Inn and was investigated for performing improper magic on goats. (This was confirmed by J.K. Rowling in an online chat at The Leaky Cauldron.)
Helen McCrory was originally cast as Bellatrix Lestrange, but dropped out due to her pregnancy. She was replaced by Helena Bonham Carter. In 2007, McCrory was cast as Bellatrix's sister, Narcissa Malfoy, in Harry Potter and the Half-Blood Prince.
Much of the Atrium set was modeled after the London Underground.
Lavender Brown has been portrayed by an uncredited black girl in the previous films. In this film, she turns white.