Rowling responded to a fan's question on Twitter to say that the experience of writing a crucial moment in the series brought about floods of tears.
And it is perhaps unsurprising to find it's within the series' climactic final book 'The Deathly Hallows'.
Chapter 34 Deathly Hallows 'The Forest Again' https://t.co/DniVfbTjKT— J.K. Rowling (@jk_rowling) October 17, 2015
So did I, but it was the culmination of 17 years' work and the most cathartic piece of writing of my life. https://t.co/XG0srTyVIE— J.K. Rowling (@jk_rowling) October 17, 2015
Chapter 34 includes the crucial moment Harry knows he must die in order to end Voldemort's reign of terror, and when he returns to the Forbidden Forest besides Hogwarts to confront his lifelong foe.
Rowling wrote the final pages of the book in a hotel room in Edinburgh and the publishers reportedly spent millions attempting to protect the final storyline.
She's previously told of her high emotions while finishing the final book, telling an interviewer she "really, really cried" whilst writing her favourite chapter in particular.
The author is incredibly active online, regularly conversing with dedicated 'Potter' fans around intricate details of the series.