1. Over 7,000 girls auditioned for the part of Lavender Brown.
2. The final seven had to improvise on screen with Rupert Grint over a period of five hours.
3. Jessie Cave, who eventually landed the role, said she wouldn't have minded if she didn't get the part because the screen test was exciting enough.
4. The scene in which Ron squeezes in between Ginny and Harry to break them up and offer them a pie had to be filmed more than 20 times because Daniel Radcliffe and Rupert Grint kept bursting into fits of hysterics.
5. Radcliffe was gutted that Quidditch reappeared in Half Blood Prince because it meant that he had to sit on a broom for five hours a day when they filmed match scenes again.
6. Grint, however, was delighted because he didn't get to play Quidditch the first time round.
7. He had to do a few weeks of stunt training before being allowed to film any scenes though.
8. A giant swing was used to send actors flying across the room for scenes in which Quidditch players fells off their brooms.
9. After the film came out, Radcliffe said he hated watching himself in it because his acting is very "one-note".
10. He has also claimed that he sometimes turned up to the Half Blood Prince set drunk.
11. David Yates, the film's director, sites Rembrandt as an influence for Half Blood Prince's colour palate and lighting.
12. Tom Felton said that whenever David Yates got a shot he liked, he got so excited that he jumped around like a 5-year-old.
13. Over 80 sets were used for this film.
14. And there were 1,500 special effects used.
15. Hero Fiennes-Tiffin, who plays a young Tom Riddle, is the nephew of Ralph Fiennes, who plays Voldemort.
16. Jim Broadbent, who plays Professor Slughorn, played Tom Felton's on-screen dad in The Borrowers when Felton was 7-years-old. Felton didn't think Broadbent would remember him, but on his first day of filming Broadbent called Felton "Peagreen", which was his character's name in The Borrowers.
17. Dave Legeno, who plays Greyback, was a professional cage fighter IRL.
18. Bill Nighy was hired to play Rufus Scrimgeour during casting for Half Blood Prince, but the character got cut from the film. Instead, they just gave him the same role for Deathly Hallows: Part 1.
19. A total of 27 owls appear in Half Blood Prince.
20. And every owl had a minimum of three doubles on set at any time.
21. All of the owls came from animal sanctuaries.
22. Bonnie Wright was really into art, so she spent a week doing work experience in the art department. She helped decorate the Weasley house, and said it was funny filming around objects that she'd helped make.
23. In the book the Brockdale Bridge, not the Millenium Bridge, is destroyed by Death Eaters.
24. In the first take of the scene in which Hermione hits Harry on the head in the library, Watson did it so hard that the scene had to be re-filmed because you could hear the bang against Radcliffe's head.
25. The cauldrons that you see in the Potions classroom are remote control operated.
26. And the Dark Mark is actually a transfer tattoo.
27. Four of the film's make up artists spent five hours working on "tattoo man", who was supposed to appear on Knockturn Alley. Sadly, he got cut from the film.
28. The knitting needles that knit on their own aren't CGI; they're actually operated by a custom built machine.
29. Luna Lovegood's Christmas dress was based on a Christmas tree and took two months to make.
30. Evanna Lynch made her own corsage featuring a hare, Luna's patronus, on it. It took her 45 minutes to do.
31. In the first draft of the script, Dumbledore talks to Harry about having a crush on a girl when he was younger. JK Rowling cut the line pointing out that Dumbledore wouldn't have said have said it because he is gay.