Speaking as part of a bBC documentary set to air this weekend, Rowling admitted that Professor Sprout – played by Miriam Margolyes in the films – holds a particularly special place in her heart.
She recalled staying at a friend’s house in 1990, after having started work on the first book about six months earlier.
When her friend went to bed, she stayed up to sketch her character whilst watching a film.
“The reason I can be incredibly precise about when I drew this is because at some point during the time I was watching that movie and drawing this picture, my mother died 250 miles away,” she said.
“I got the phone call the next day to say that she had died. So, this obviously means a great deal to me, this picture.”
Sprout, she added, is “the most maternal or paternal” of Hogwarts’ four heads of houses.
Harry Potter and the Philosopher’s Stone was first published on June 26, 1997 in the UK; and a great deal of commemorative 20th anniversary celebrations have been taking place throughout the year.
This new documentary goes hand-in-hand with a new exhibition at the British Library.
The final film in the series was released in 2011, and its lead star – Daniel Radcliffe – recently released his latest film, Jungle.
Emma Watson, meanwhile, starred this year in Beauty and the Beast.
A History of Magic airs on BBC Two on Saturday at 9pm.