In order to acquaint himself with his three lead actors and actress, director Alfonso Cuarón 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 a sixteen-page essay. Daniel Radcliffe, like Harry, wrote a simple one-page summary, and Rupert Grint, like Ron, never even turned his in.
Director Alfonso Cuarón had never read the Harry Potter novels, nor seen the first two movies, when he was offered the job of director. It was Guillermo del Toro who convinced him, saying: "Don't be stupid; read them immediately."
A clause in director Alfonso Cuarón's contract forbade him from cursing in front of the kids on-set.
Sir Ian McKellen turned down the role of Dumbledore. Having appeared as Gandalf in The Lord of the Rings, he said, "I had enough trouble living up to one legend. Two would be too much to hope for." He also stated it would have been inappropriate to take Richard Harris' role, as the late actor had called McKellen a "dreadful" actor.
During filming, all of the pockets on Tom Felton's robes were sewn shut to prevent him from sneaking food on-set.