J.K. Rowling: The epilogue of the seventh book is a very clear pointer as to where I was interested in going. It’s very obvious from that epilogue that the character I was most interested in was Albus Severus Potter. And you see Scorpius on that platform.”

“I’ve been awake since 4am. We were in the theatre last night and I saw a scene that’s very close to my heart, in costume, on the set and it was quite overwhelming.”

You can probably imagine I have been asked to do something else with Harry Potter five times a week ever since the series ended. Sonia [Friedman, producer] just wanted to explore a theatrical production and I knew her by reputation obviously and thought I would really like to meet her and hear what she had to say.”

“Jack [Thorne, playwright] and I are similar in many ways. We’re both, notwithstanding how chirpy we are being right now, quite introverted people who are very happy alone in a room, and there are many parallels in our working practices and I felt like he was one of my tribe.”

“What Jack says about ET… this is why he is the right man for the job, because he just gets it. That’s pitch perfect. The big reason why people loved Potter was that it felt like it could be. That sense that there is more to the world. Just on the other side. Even within touching distance. There’s more. It is the promise of another world and it doesn’t have to be a magical world but to a lonely child or an insecure person or anyone who feels different or isolated, the idea of having a place where you do belong is everything.”

Yet having met her, it’s clear that Harry Potter has never really left her. “It was 17 years and just because I’ve stopped on the page doesn’t mean my imagination stopped,” she says. “It’s like running a very long race. You can’t just stop dead at the finishing line. I had some material and some ideas and themes, and we three [she nods at Tiffany and Thorne] made a story.”

It has been almost a decade since she put pen to paper for the final book in the sequence. “But I carry that world around in my head all the time,” she acknowledges. “I am never going to hate that world. I love that world. But there are other worlds I want to live in too. To be perfectly honest, I just feel if I enjoy it, I’ll do it – and if I don’t, I won’t.”

In fact, with the play and the screenplay, 2016 has turned out to be what she describes as “such a wizardy year”. “I always said never say never, and the reason I said that was truthfully that I did have this residue in my head in both directions – in Fantastic Beasts…, which is going back, and in this play, which is going forwards. So I still had this material in my head.

“I kept being asked whether I would make a musical and I don’t like musicals,” she says, grimacing. “Theatre, on the other hand, I love. I find it a seductive world – there is nothing like seeing an actor perform live. But I had never had anyone approach me or propose anything that excited me like this.

“I think that, as a theatrical experience, as a play, it will be unlike anything people have seen before. And once people have had this theatrical experience, they will understand why this was the perfect medium for the story.”

On Noma Dumezweni's casting of Hermione Granger and worldwide reactions:

“With my experience of social media, I thought that idiots were going to idiot,” she says. “But what can you say? That’s the way the world is. Noma was chosen because she was the best actress for the job. When John told me he’d cast her, I said, ‘Oh, that’s fabulous’ because I’d seen her in a workshop and she was fabulous.”

“I had a bunch of racists telling me that because Hermione ‘turned white’ – that is, lost colour from her face after a shock – that she must be a white woman, which I have a great deal of difficulty with. But I decided not to get too agitated about it and simply state quite firmly that Hermione can be a black woman with my absolute blessing and enthusiasm.”

“I’ve been through this many times,” says Rowling. “And I hope we get there without any major spoilers, purely because people will have an amazing experience if they don’t know what’s coming.

“Generally speaking, Harry Potter fans are a community, they have each other’s backs, and they want to have that mystery and the sense of surprise. So we’re hopeful. But it won’t be the absolute end of the world. We’re not going to be throwing tantrums about it but we hope for the audience’s sake that we can get there.”