It took six months to train the owls to carry the letters. Even then it turned out that the letters, each one hand-crafted, were too heavy to carry very far and they had to be remade.
Rowling was offered the chance to play Lily Potter in the Mirror of Erised scenes (cos she’s Harry’s mother, geddit?) but she turned the offer down.
Tim Roth (Pulp Fiction, Reservoir Dogs) was the first choice for Snape but he turned it down to appear in Tim Burton’s Planet of the Apes.
When Alan Rickman got the part, Rowling gave him “one tiny, little piece of left-field information” to let him know that the character was not as nasty as he seemed. Rowling later revealed that she told him of Snape’s undying love for Lily.
The last scene of the movie – where Harry, Ron and Hermione leave Hogwarts on the Hogwarts Express – was actually filmed on the first day.