In my humble opinion, Tom (Styles) and Marion (Corrin) are perfectly cast, and while I am less familiar with David Dawson, he also seems to be a promising Patrick. What was your reaction to the Harry Styles casting in particular?

Oh, I have a confession here. As a middle-aged bookish type, I wasn’t that familiar with the work of Harry Styles. I know, I know: I can only apologize. So when Michael Grandage, the director, told me that he might be playing Tom, I knew it was big, but I had no idea quite how big! Needless to say, as soon as I left that meeting, I googled him and I realized how incredible this was. I think I shouted something unrepeatable, and then I ran to get my train. I wasn’t late, but I was so exhilarated I just had to break into a run—unheard of for me—and I was probably laughing like a madwoman at the same time. So many of his fans are getting in touch to say they’ve enjoyed the book, which is amazing. I am totally delighted with the casting. Rupert Everett, Linus Roache, and Gina McKee [who play older versions of Patrick, Tom, and Marion, respectively] are acting royalty. Emma Corrin is incredible. They have such strength and such delicacy. And I think David Dawson fits Patrick so well. Looks-wise, he’s very, very close to how I imagined Patrick to be.

The book is absolutely beautiful. I read that you were inspired by E.M. Forster’s longtime affair with a married policeman, and the triangle between them and said policeman’s wife, May. How did this real-life story spark inspiration for you?

Thank you! I’ve loved Forster’s work for a long time. His work—and his life—were quietly revolutionary in a way I really admire. I adore his humor and his warmth and his satirical eye. I had an idea to write some sort of fiction about Forster himself. I researched his biography, and that was where I read about May Buckingham, the wife of Forster’s lover of 40 years, the policeman Bob Buckingham. And I read that, despite the struggles the three of them had in sorting out how this triangular relationship might work, particularly in an era when homosexuality was an imprisonable offense, May was actually the one who was there, holding Forster’s hand, as he died. I began to wonder how that came about. At the same time, I’d wanted to write about my adopted hometown of Brighton for years, so I thought, perhaps I could transpose a similar story to that location.

You did not come of age in the ’50s. How did you learn about that era, and the attitudes toward queer people then?

I read novels and newspapers and memoirs of the time, particularly Peter Wildeblood’s searing Against the Law, but also work by J.R. Ackerley, Harry Daley, Colin Spencer, Rupert Croft-Cooke. I watched films and looked at photographs. My parents did come of age in that era, so I spoke to them. The most useful book was a local history project called Daring Hearts: Gay and Lesbian Lives in 1950s and 1960s Brighton. Hearing the beautiful voices of the men and women who’d lived and loved through that repressive era was inspirational.

There’s so much to unpack in the central “love triangle.” It’s intriguing that while Patrick and Marion both resent each other, at times, because of Tom, they bond as well. In a way, both Tom and Marion “date” Patrick. What were some of your favorite tensions and dynamics to explore between the three characters?

I think you’re right—Marion and Patrick do “date,” in a way. Patrick is sophisticated and has access to a cultural world that’s much harder for Marion and Tom to reach without him. I did enjoy teasing out the ways in which Marion and Patrick are similar, and the ways in which class had, and still has, such a stranglehold on their lives. As a lower middle-class woman of her era, Marion’s sexual longings are also taboo. Of course, she is partially blind to the ways in which Patrick’s situation is so much more dangerous than hers. I loved exploring the drama of the jealousies and the longings, and the way in which they both create a fantasy of Tom.

Books are often better than their movie adaptations, and movies take liberties. How are you feeling about the movie coming out next year?

I think I’ll answer this with a phrase a teacher used about me in my primary school report, and I’ve been pleased by ever since: quietly confident.

This interview has been edited and condensed.