De acordo com a Bloomsbury, ela teve a ideia em uma estação de trem.

A biografia de JKR na Bloomsbury diz que ela teve a primeira ideia da série Harry Potter em uma viagem para a Estação Kings Cross.

J.K. Rowling first had the idea for Harry Potter while delayed on a train travelling from Manchester to London King’s Cross in 1990. Over the next five years, she began to plan out the seven books of the series. She wrote mostly in longhand and amassed a mountain of notes, many of which were on scraps of paper.

Para partes específicas da história, ela às vezes cita coisas específicas, como Hermione sendo uma versão mais jovem de si mesma, ou Lockhart sendo alguém que ela conhecia. Ela também mencionou que certo folclore e mitologia a inspiraram, como "O Conde do Conde", inspirando a história dos irmãos Peverell.

Mais genericamente, ela disse que não sabe de onde vem sua inspiração, em esta entrevista .

THE question you are most frequently asked as an author is: "Where do you get your ideas from?" I find it very frustrating because, speaking personally, I haven't got the faintest idea where my ideas come from, or how my imagination works. I'm just grateful that it does, because it gives me more entertainment than it gives anyone else.

Várias vezes, J. Rowling foi questionada em entrevistas, mas nem ela realmente sabe. O mais relevante é provavelmente uma entrevista um pouco ruim de 1999 :

What inspired the Harry Potter series?

I really don't know where the idea came from. It came into my mind when I was on a train to London. Harry as a character came fully formed, as did the idea for his sidekicks, the characters of Ron and Hermione. It started with Harry, then all these characters and situations came flooding into my head. It was an excitement I'd never known before. The characters arrived in 1990, but it took me six years to write the book.

Esta pergunta do FAQ em seu site antigo também é relevante:

Where do you get your ideas?

This is the question every author is asked most. The answer is ‘out of my head’, but people don’t seem very satisfied with that, it’s too boring, even though it’s true. I suppose an idea might be triggered by something you see or hear, though I can’t remember an instance of that happening to me. For me, the most idea-producing-situation is to be sitting in a fairly quiet corner of a café, looking down at a nice blank sheet of paper, with a big mug of tea slightly to the left and a new pen clutched in my right hand.