The first West End production of Birdsong opened in 2010, and is still going strong. Why do you think it has proved such a success?

Mostly it lies with the book, which is an extraordinary piece of work. It is a wonderful and immersive way to imagine what it might have been like to be involved in the First World War. We can really get inside the heads of the characters that Sebastian has created, and understand what it might have been like for ordinary people.

What prompted you to think of adapting the novel?

I first read it when I was 17 and fell absolutely in love with it. I loved the character of Jack Firebrace and the tunnellers, but also there is something very unusual and arresting about the love story. I found it a very powerful book, and I could see some of the scenes working on stage. I got in touch with Sebastian and was put on to his agent, who found out that the stage rights were available. It went from being a childhood dream to being something that looked frighteningly possible. Then, this being theatre and a very popular novel, it took some time to persuade everyone to let me have a go. I wrote a first draft and it went from there.

What was it like to meet Sebastian?

I was terrified. When you meet someone that you really admire, you think “what if they’re not very kind or they don’t like you or think you’re an idiot?” Fortunately, I knew from having read his novels that he must be a very compassionate man, and it turned out he was. I was very relieved. He was very kind to me, very nice, very polite. I loved him instantly.

You obviously got on well, as you have been working on two other screenplays with him?

He asked me about adapting another of his novels for the screen, then another one. It is lovely. It is very flattering and very encouraging and very reassuring. I know how much a novel can mean to people. We all feel very possessive of novels we love.

There is a new cast for this year’s production. How does it feel to see new actors interpret your work?

It is strange. You get attached to the last cast but it’s also really interesting. I learn a lot from it. If something’s not quite working with one actor, you can try it out with another actor. And I am constantly fleshing out the parts and making them as real as possible.

Do you think the fact that 2014 marks 100 years since the outbreak of the First World War gives the show extra poignancy?

I hope so. I think it’s really important that we do remember these stories and remember what people 100 years did for us. It isn’t just something that happened in history. Huge mistakes were made and huge sacrifices were made, and we must remember and honour those, lest we forget. Humans do keep seeming to make the same mistakes again and again. We must keep these stories alive.

Did you ever imagine this could be your full-time job?

No. My mum works in a primary school and my dad drives buses. It was not something that was in the world where I grew up. It never occurred to me that people could make a living as a writer. I am shocked and thrilled that I am allowed to keep doing this.

Do you enjoy being in the theatre world?

I love it, although it is a difficult world as there’s lots of fighting going on and I’m not someone pushy. I don’t want to go and schmooze and network. You hope you can do your own thing so hopefully someone will put it on, but sometimes it’s a bit trickier than that.

Birdsong will be at Tunbridge Wells’ Assembly Hall Theatre from Monday, February 24, to Saturday,
March 1. Tickets cost from £19.25. There are matinee performances on the Wednesday and Saturday. Call 01892 530613.