"In Third Year, we were taught about Animagi," James explains. "Witches or wizards who can — with a bloody lot of hard work and extremely careful practice — turn themselves into an animal.

"They only get the one form, and it's quite common for that form to be the same as the witch or wizard's patronus because it's a reflection of themselves and their inner nature.

"Anyway, that's where I got the idea, really. From Third Year."