1. What are you writing right now?

I’ve just finished the fourth Galbraith novel, Lethal White, and I’m now writing the screenplay for Fantastic Beasts 3. After that I’ll be writing another book for children. I’ve been playing with the (non-Harry Potter/wizarding world) story for about six years, so it’s about time I get it down on paper.

2. What is a typical writing day?

I try to start work before 9am. My writing room is probably my favourite place in the world. It’s in the garden, about a minute’s walk from the house. There’s a central room where I work, a kettle, a sink and a cupboard-sized bathroom. The radio is usually tuned to classical music, because I find human voices the most distracting when I’m working, although a background buzz, as in a café, is always comforting. I used to love writing in cafés and gave it up reluctantly, but part of the point of being alone in a crowd was being happily anonymous and free to people-watch, and when you’re the one being watched, you become too self-conscious to work.

The earlier in the day I start, the more productive I am. In the last year or two I’ve put in a couple of all-nighters on the screenplays for Fantastic Beasts, but otherwise I try and keep my writing to the daytime. If I’ve started around nine, I can usually work through to about 3pm before I need more than a short break. During this writing time, I generally manage to drink eight or nine mugs of tea. Being incredibly clumsy, prefer eating things that won’t ruin the keyboard when dropped. Popcorn’s ideal.

3. You have collaborated on several projects. How does that work?

By nature I’m quite solitary, so novels and I suit each other perfectly, but the collaborations I’ve been involved in have been pure joy, mainly because of the people involved.

For ten years I said no to proposals to adapt Harry for the stage, usually as a musical, and using the existing books. So when I met producers Sonia Friedman and Colin Callender, I wasn’t sure what I was going to hear. I only knew that they wanted to do something new, which was intriguing, because I had no desire to go endlessly back over old ground.

I couldn’t have asked for better collaborators on the stage play than John Tiffany (director) and Jack Thorne (writer). Incredibly, John and I knew each to say hello to years ago, when I used to write in the café at the Traverse Theatre in Edinburgh. When we met for the first time about Cursed Child, I stared at him, thinking, he looks so familiar, where have I met him? And he told me, and the whole thing felt oddly fated.

I told John and Jack what I thought had happened to Harry, Ron and Hermione in later years, explained how focused I was on Harry’s son Albus, who’d been given the burden of not one, but three legendary names, and together we created the story that Jack wrote.

I have so many wonderful memories of the earliest rehearsals, of seeing the costumes and illusions for the first time, but what I remember most fondly about the three of us working together is the laughter. I loved the process from beginning to end.

I particularly remember the first full dress rehearsal I watched. By this time I knew the script backwards, had heard it read all the way through and watched individual scenes acted, but nothing prepared me for seeing it in its entirety, in the theatre. I found it incredibly moving and it brought back a tsunami of memories about the seventeen years I spent creating the characters and writing the Harry Potter books. John and Jack did a superb job. Very few people have come inside the world with me and it creates a particular bond.

The big difference between theatre and movies for me is scale. When I go down to WB Studios Leavesden and see a thousand people at work on Fantastic Beasts, building sets, making costumes, doing digital effects, making models and props and all the hundreds of other things that go into making a movie, it can feel utterly overwhelming. Terrifying thoughts run through your mind, such as, I must not break an arm, because all these people’s jobs depend on me getting the screenplay finished.

However, at the heart of the process is a very similar collaboration to the one I had on Cursed Child, this time with David Yates, the director, and Steve Kloves, who was the writer on seven of the eight Potter films and is a producer on Beasts.

In spite of the fact that I’d watched Steve close up for all those years, I found screenwriting utterly different from novel writing and very challenging at first. Basically, I learned how to write a screenplay as I went along, knowing that the movie was definitely going to be made, which is, to say the least, atypical. Steve gives great, pithy notes. The one that made me laugh longest was when I had a character in a cut scene in an early draft say, ‘They’re children!’. He said, ‘Yeah, unless we’ve got the casting badly wrong, that’ll probably be obvious.’

David knows the world of Potter intimately now, after directing four of the eight original movies. I love working with him. I learn a lot just listening to him talk about images. Even though I have a highly visual imagination, I’ve had to learn just how much can be said onscreen without a word, and David and Steve have taught me that.

The thing with movies is, however frustrated you get with the screenwriting process, and right at the moment when you think ‘never again, this is too hard’, you go down to the film set and join in with one big glorious game of pretend, with the world’s best pretenders saying your words, and dressing out of the most fabulous dressing up box, and what with the lights and the smoke and the music you’re suddenly in love with the process all over again.

4. What exactly is your role as producer? How much say do you have in the look and feel of the films?

Warner Bros and David Yates, the director, have always let me have my say, though not necessarily the final word. That’s true of all the producers, of whom I’m only one: our input is taken seriously but it is very much a collaborative effort. The director is ultimately responsible for everything that’s seen on the screen. As the screenwriter, the majority of my input comes at an earlier stage.

5. Do you write for readers or for yourself?

This is a tricky question in some ways, because a writer who truly only wrote for themselves probably wouldn’t try and get published. At the same time, I agree with Cyril Connolly’s words: ‘Better to write for yourself and have no public, than write for the public and have no self.’

I certainly write ‘for myself’ in the sense that I have to write. It’s almost a compulsion. I need to do it. I don’t feel like myself if I’m not writing regularly, and I feel restless and odd if I have nothing to write, which these days is never, because I’ve got so many different projects on the go, by choice. I also write for myself in that I need to feel excited about a story to want to capture it on paper. I’m afraid I couldn’t write anything just because I knew people wanted it. The impetus always has to come from within.

On the other hand, no story lives unless someone is prepared to listen. As a writer, your highest aspiration is to touch people, to connect, to amuse or console. What could be more wonderful than hearing that your book helped somebody through a tough time? I think of the times when books have been my best consolation and source of strength, and I’m proud beyond words when I hear that anything I wrote did the same for other people.