”You think you’re a Harry Potter fan until you meet a Harry Potter fan,” says John Tiffany. “They take it to another level.”

Harry Potter and the Cursed Child, New York. Photo © Matthew Murphy

With hordes of aficionados breathing down your neck, creating a new Harry Potter story might appear a daunting task. In actual fact, the British director was surprisingly cool when first approached to work on a sequel to the seven-book series, but perhaps that was partly because of an early encounter with author JK Rowling.

“My first ever job was as an assistant director at the Traverse Theatre and we were one of the first places in Edinburgh to serve lattes and cappuccinos,” he recalls. “Now and again I would see this woman with a pram and she would sit there for two or three hours, clearly writing a book. We didn’t really speak, but we used to say ‘hello’. Now and again she would ask if I minded her sitting here for this long and I would say absolutely not, no. Eighteen months later, The Philosopher’s Stone came out...