1. When Alfonso Cuarón became Prisoner of Azkaban's director, he asked Daniel Radcliffe, Emma Watson, and Rupert Grint to write essays on their characters. Radcliffe wrote a page, Watson wrote 10, and Grint never managed to hand one in.
2. Watson had a Hermione moment when her essay was so good that Cuarón read it aloud to Radcliffe and Grint.
3. Cuarón inspired both Radcliffe and Watson to study Spanish at school.
4. But he almost turned down the opportunity to direct Prisoner of Azkaban because he was worried about being able to express his own vision while working as part of a franchise.
5. David Thewlis, who played Professor Lupin, had previously auditioned for the role of Professor Quirrell in Philosopher's Stone.
6. Thewlis hadn't actually read Prisoner of Azkaban until he got the part.
7. Emma Thompson, who played Professor Trelawney, was one of Watson's favourite actors.
8. Whilst Gary Oldman, who played Sirius Black, was one of Radcliffe's.
9. Oldman said that he felt he had to be good on set because he knew he was Radcliffe's idol.
10. He gave Radcliffe a bass lesson on set.
11. And he bought his own children to see the set.
12. But Watson hadn't heard of Oldman before they started filming.
13. Prisoner of Azkaban was Watson's favourite of the Harry Potter books.
14. The whole cast and crew moved up to Scotland for filming because Prisoner of Azkaban contained so many outdoor scenes.
15. But it rained so much that producers had to order helicopters to deliver gravel to prevent the cast from falling over.
16. Before being created in CG, the Dementors were made as puppets.
17. In the scene in which Harry and Sirius face a Dementor by the lake, Radcliffe accidentally forgot to breathe. He started hyper-ventilating and then almost fainted.
18. It took Thewlis six hours to be transformed into a werewolf.
19. The contact lenses he wore actually did mean that he couldn't see.
20. During filming, Michael Gambon, Alfonso Cuarón, and Alan Rickman played a prank on Radcliffe: They planted a fart machine in his sleeping bag for the scene where the entire school sleeps in the Great Hall, because he was positioned next to a girl he fancied.
21. Radcliffe tried to stay in character.
22. Before filming difficult scenes, Radcliffe would listen to music to get him in the right frame of mind.
23. Like Ron, Grint is scared of spiders.
24. During filming, Buckbeak was just a beak on a stick.
25. In the first animations, Cuarón described Buckbeak as being "like a puppy". He then asked animators to make him more grown up.
26. Buckbeak was made of lycra and covered in individually-made feathers.
27. The Knight Bus was so top heavy that four tons of weight had to be added to its base so it didn't fall over.
28. The bus they used for filming could move twice as fast as a London bus.
29. They filmed its scenes by having the other cars in the road drive really slowly.
30. And they built a set for the bus's interior on top of a moving plate, so that they could film everything sliding around under controlled conditions.
31. The Shrieking Shack scenes were particularly hard to film because there wasn't meant to be any actual light. As a result, the directors built gaps in between the wooden panels on the walls to let light in.
32. One cold day, Radcliffe was doing press for the film in New York when he spotted a girl standing outside his studio wearing nothing but a Harry Potter towel. She was holding up a sign that said, "Nothing comes between me and Harry Potter".
33. Matthew Lewis, who played Neville Longbottom, started wearing false teeth, plastic behind his ears to make them stick out more, and shoes two sizes too big in Prisoner of Azkaban.
34. Harry Melling, who played Dudley Dursley, had to start wearing a fat suit.
35. And Timothy Spall, who played Peter Pettigrew, wore false teeth and nails.
36. An awkward hug between Hermione and Ron was cut from the film.
37. At the premiere, everyone cheered when Hermione hit Draco.
38. After filming finished, Watson kept her wand, cloak, and the time turner.
39. She said she'd put the money she earned in the bank.
40. Whereas Radcliffe said he'd spend his on CDs.
41. And finally: During filming, the rat who played Scabbers peed on Grint.