Trivia: In order to acquaint himself with his three lead actors, director Alfonso Cuaron had each of them write an essay about their characters, from a first-person point of view. Emma Watson, in true Hermione fashion, went a little overboard and wrote an 16-page essay, Daniel Radcliffe wrote a simple one page paper, and Rupert Grint never even turned his in, as he said that is what Ron would have done.
Trivia: There are several scenes on the staircases where you can see an 'overanxious' knight in the background, fighting and running around and such, most visible when the fat lady is discovered missing. This knight, though not prominent in the film, is mentioned several times in the books and initially helped Harry, Ron, and Hermione to find their first divination lesson. His name is Sir Cadogan and he was originally going to be played by Paul Whitehouse.
Trivia: When Harry goes to Honeydukes, the candy shop in Hogsmeade, there is a scene that shows him entering through a hidden hole in the floor. On the left you can see a lot of sugar skulls. This is a typical Mexican candy that is very popular around the first days of November, when Mexicans celebrate "The Day of the Dead" (The film's director, Alfonso Cuaron, is Mexican).
Trivia: For those who haven't read the book, Hermione's cat Crookshanks, is no ordinary feline. He is able to communicate with Sirius Black's animagus form and helps him sneak into Hogwarts. This extraordinary ability is explained further in J. K. Rowling's book, Fantastic Beasts and Where to Find Them, where it's noted that Crookshanks is part kneazle.
Trivia: When Lupin is turning into a werewolf, Sirius says to him, "Did you take your potion tonight?" He is referring to a wolfsbane potion, which is not included in the film. According to the book, if a werewolf drinks the potion on the day before and the day of a full moon, he can keep his mind when he transforms. Lupin forgot to take it because he saw Harry with Sirius in the Shrieking Shack (on the Marauder's Map). Therefore, he became a fully-fledged monster.
Trivia: Director Alfonso Cuaron makes two cameo appearances. The first is a man in the Three Broomsticks, Madame Rosmerta's pub, who holds a newspaper and what appears to be one candle which becomes two and then three, with quick finger maneuvering. The second is as one of the Hogwarts paintings when Harry is looking for Peter Pettigrew with the Marauder's Map.
Trivia: During Hagrid's Care of Magical Creatures lesson, just before Harry approaches Buckbeak in the paddock Beaky has a 'potty accident.' Manure is seen dropping from his hindquarters and can be heard thudding onto the ground. This is computer generated - the CGI artists were having a bit of fun and wanted to add a touch of realism. (Commentary on DVD 2). (00:34:00)raywest