Trivia: The costume department tried in vain to produce Luna's radish-like earrings. Evana Lynch made her own - they're the ones she wears in the film.
Trivia: The director and other persons of importance had cut Kreacher out of the film. JKR told them that she couldn't tell them what to do, but the final film wouldn't make sense if they didn't introduce Kreacher in this one. Five minutes later, Kreacher was back in.
Trivia: 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.)
Trivia: 15,000 girls auditioned for the role of Luna Lovegood.
Trivia: Over 5000 kittens were filmed for the decorative plates in Umbridge's office.
Trivia: At the end of the movie, director David Yates appears as one of the magical moving portraits hanging up in Hogwarts.