One question I’ve often been asked is whether I wrote Before I Go to Sleep with an eye on it being turned into a film. The truthful answer is that when I was writing Before I Go to Sleep I spent my time wrestling with the plot, trying to bully my characters into at least trying to do what I needed them to do, and kicking myself for even trying to write a novel about a woman with no memory in the first person.

I was too busy trying to get the book to work – to make sense and be enjoyable – to worry about what might happen once it was finished, and though I did think that maybe, just maybe, I might see it published, the thought that it might actually be turned into a movie, with actors and a set and someone shouting ‘Cut!’, did not cross my mind.

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But now, here it is. Before I Go to Sleep, produced by Liza Marshall and Scott Free, directed by Rowan Joffe, starring Nicole Kidman, Colin Firth, Mark Strong and Ann-Marie Duff. Not only People I’ve Heard Of, but People Who Have Won Awards. I still have to pinch myself.

The whole process has been surreal. It began with an email, just after my agent had sent the book out to publishers. The email was unexpected and terribly polite. “We love your book,” it said, “and really want to help bring it to the screen.”

The name Ridley was mentioned, and when I checked the email address I could see that it was indeed from someone at Scott Free. Once I’d recovered I sent it to my agent, and a few days later we got more information about what they were offering.

Rowan Joffe, who’d won a Bafta and had just finished making Brighton Rock, had been given the book by Liza Marshall who loved it and thought it should be their next project. He agreed, and they had some really exciting ideas about casting.

They wanted to set up a meeting, so that I could meet them and make sure I was happy with what they wanted to do with the story. Would that be alright?

I wasn’t going to say no, was I? I went to meet Rowan and Liza, still not quite able to believe this was really happening, but by now slightly more wary. I’d heard so many stories of films that had nearly been made, falling through at the last moment because of a change in the tax law in Azerbaijan, or (more often) the whims of a capricious leading man.

Plus there was also the danger that Rowan, who was writing the screenplay, would want to mangle my story, set it in space, or remove the whole problematic ‘memory loss thing’.

As it turned out, he didn’t. His enthusiasm for the book was almost overwhelming, and his knowledge of it encyclopaedic (I swear that even at that first meeting he knew more about how the story fitted together than I did).

Liza, who would produce the film, was equally passionate about the film they would make together, and so from the word go I knew I was in safe hands. I agreed to sign.

S.J. Watson has relished every minute of seeing his book Before I Go To Sleep transformed into a film. Photo: Supplied

It’s an odd thing, signing a film deal. Essentially I gave them the rights to do pretty much whatever they wanted with my story and my characters. But I told myself I would trust my initial thoughts and just let them both get on with it (I’ve also heard stories about films being ruined by interfering authors who just wouldn’t stay out of the way), unless they suggested something I really disagreed with.

Rowan and Liza, for their part, were keen to run everything past me, keeping me up-to-date on casting (Nicole Kidman? Colin Firth? I thought someone was playing an elaborate practical joke), sending me the script as soon as they had a draft (I loved it straight away, and could see instantly the reasons for the changes Rowan had made – endless shots of someone writing in a diary just wouldn’t make thrilling viewing, for example), even asking my opinion on costumes and locations (in particular, where would Christine and Ben be living? A terrace in Crouch End was right for the book, but would it be cinematic enough for the film they wanted to make? ‘Something like the house in Psycho?’ I said, excitedly, to which Rowan replied, quite rightly, ‘Well, maybe let’s not go too far…’).

There wasn’t a single point on which we disagreed.

And then, one day in February 2013, I found myself travelling across London to Twickenham Studios to visit the set. The day has to rank as one of the most extraordinary of my life. When I write, of course, I imagine the scenes in great detail – I think every writer does – but here I was, standing on the set, inside Christine’s house. It felt real. I could reach out and touch the computer in Ben’s office, the phone that Dr Nash rings her on, see the bed where she first wakes up, believing herself young and foolish.

It was my imagination, suddenly real, solid, tangible. When I saw Nicole Kidman and Colin Firth standing just a few feet away, looking exactly how my characters ought to look, any doubts I might’ve had about the film (negligible as they were) vanished instantly.

They completely inhabited Ben and Christine and were bringing them to life in the most unbelievable way, right in front of my eyes. “Will you come and visit again?” said Liza when I left that day. “You try stopping me,” I replied.

And now the film is finished. I’ve seen it (four times, so far) and I’m delighted. Rowan has made a stunning thriller, centred on some amazing performances, with a real heart. I can’t take the credit for the film, and still can’t quite believe that it started in my head, but now the story has another life. It’s been an amazing journey for me, but one moment stands out.

I was watching Nicole Kidman filming a scene and it wasn’t quite working. “It’s the line,” she said. “Christine wouldn’t say that.”

I hadn’t spotted it, but when she offered her alternative the whole scene suddenly clicked into place. It was incredible to watch, and I remember thinking that she now knew Christine at least as well as I did. The character that began in my head, all that time ago, now has another existence, another incarnation, and I couldn’t be happier.

Christine wakes in a strange bed beside a man she does not recognise. In the bathroom she finds a photograph of him taped to the mirror, and beneath it the words ‘Your husband’. Each day, Christine wakes knowing nothing of her life. Each night, her mind erases the day. But before she goes to sleep, she will recover fragments from her past, flashbacks to the accident that damaged her, and then—mercifully—she will forget. Chilling, exquisitely crafted and compulsively readable, S. J. Watson’s debut novel Before I Go to Sleep is a psychological thriller of the highest order. It asks primary questions. Are there things best not remembered? Who are we if we do not know our own history? How do we love without memory? 

Buy Before I Go To Sleep here.