SJ Watson: 'I still can't quite believe that Before I Go to Sleep started in my head'

Watson was working part time in the NHS when his debut novel, Before I Go to Sleep, became a bestseller. Now it's a Hollywood movie, too. Here he recalls the whirlwind journey from children’s ward to A-list film set

Eyes wide open: Nicole Kidman and Colin Firth in ‘Before I Go to Sleep’ (Laurie Sparham)

I wrote Before I Go to Sleep in 2009. During that year I was working part time as an audiologist. For three days a week I was responsible for the assessment of babies and young children with severe hearing impairments, working with families who were often going through an incredibly traumatic time while trying to reconcile the demand to do more work with fewer resources imposed by an NHS that was only just beginning to feel the pinch. It was stressful sometimes, but I was lucky enough to be part of a supportive team and our burdens were shared.

Not so with my other job. It had always been my ambition to have a novel published, and so for four days a week I was at the dining table, hunched over my laptop, trying to bully imaginary people into doing what I wanted them to do. Some days it went well, I was thinking up twists, new scenarios, interesting ways in which to surprise the (at this point hypothetical) reader. On other days I'd sit there, kicking myself for thinking I could write a book about a woman who wakes up every day with her memory wiped clean, in the first person.

But eventually I finished it. I was happy. It worked, I liked it. So did my Mum. I thought I might be able to get it published, though I knew the odds were stacked against any new writer. Did I think it would become an international bestseller? No. Did I anticipate giving up audiology to become a full-time writer? Definitely not. Did I imagine seeing my story in the cinema? With A-list, Hollywood, Oscar-winning actors? Literally didn't cross my mind. All that stuff happens to other people. Not me.

Yet five years on, here we are. It happened. I met an agent, we got on, she liked my work. She sent it out, and publishers were "very interested indeed". I was ridiculously happy. I shared my news with my NHS colleagues, and they were ridiculously happy for me, too. And then one lunchtime, between patients, I received an email. It was unexpected and terribly polite. "We love your book," it said, "and really want to help bring it to the screen." The name Ridley Scott was mentioned.

It was a surreal moment. We hadn't even sent the book out for film. I read it over and over again, and then, once I'd recovered, I forwarded it to my agent. A few days later we got more information about what they were offering. Writer and director Rowan Joffe had been given the book by producer Liza Marshall who thought it should be their next project. He agreed, and they had some really exciting ideas about casting. Could we meet to discuss it further?

I wasn't about to say no, was I? Still feeling slightly out of my depth we had a meeting. Rowan told me all about how much he loved the book, and the ideas he had for the film he and Liza wanted to make. He'd done his homework and even then knew more about how the story fitted together than I did. He could see Christine in his mind, he had some very exciting ideas about the script. He told me had a personal connection to the material, and he really hoped we could work together. Straight away I knew I was in safe hands. I agreed to sign.

Yet I didn't get too excited. I'd heard that it takes on average nine years to turn a book into a film, and most don't get made at all. Plus there was the lingering worry that once the mysterious "studio executives" were on board the story would be mangled, vampires might suddenly appear or they'd decide to get rid of the whole problematic "memory loss thing". Rowan and Liza were keen to include me at every stage, but I decided I wouldn't get too involved. I'd just let them get on with making their film.

I needn't have worried. A script appeared, which I loved straight away. Then the cast started to come together, and things got even more surreal. Nicole Kidman? Colin Firth? Mark Strong? I thought someone was playing an elaborate practical joke. Costumes were discussed, and locations. It was decided that a terrace in Crouch End, north London, might be right for the book, but wasn't cinematic enough for the film. "Is that OK?" said Rowan, to which I replied "Yes! How about using something like the house in Psycho?" He paused then said, quite rightly, "Well, maybe let's not go too far…" There wasn't a single point on which we disagreed.

SJ Watson: 'I still can’t quite believe that it started in my head'

And then, one day in February 2013, I found myself in Twickenham Studios. The day was one of the most extraordinary of my life. Here I was, standing inside Christine's house. I could reach out and touch the computer in Ben's office, the phone that Dr Nash rings her on, the bed where she first wakes up, believing herself young and foolish. It was my imagination, suddenly made real. When I saw Nicole Kidman and Colin Firth sitting 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. "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, and I'm delighted. Rowan has made a stunning thriller, centred on some amazing performances. 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 though I kind of miss her, I couldn't be happier.

'Before I Go To Sleep' is released on 5 September; the book is published by Black Swan (£7.99)