"As the book started to take shape the characters almost seemed to develop a will of their own," says S J Watson

SJ Watson's thriller is based around a woman who suffers a head trauma and can only retain memories for 24 hours at a time. He wrote it between shifts at London's St Thomas's Hospital and it was an instant success on publication, becoming a New York Times and Sunday Times bestseller and selling millions of copies in 42 countries around the world. It won the Crime Writers’ Association Award for Best Debut Novel and the Galaxy National Book Award for Crime Thriller of the Year. And that was all before Ridley Scott’s production company secured production rights to it and turned it into a blockbuster movie (out this Friday), starring Colin Firth and Nicole Kidman. Clearly, this is a man who knows how to write a book.

Here, he tells us how he came up with the idea for Before I Go To Sleep and what he did to make it a success. He also shares a few precious tips for all you aspiring authors out there - basically, be prepared to write a lot, and badly, before you get anywhere...

How did you first come up with the concept of Before I Go To Sleep?

It sprang from an obituary I read about a man called Henry Gustav Molaison, who died in 2008. He’d been unable to form new memories since undergoing surgery for epilepsy in 1953, and it occurred to me that he must have woken up every day thinking he was still in his twenties. I thought there might be material there to write a novel, and sure enough over the following weeks more and more ideas started to develop. I think the best ideas are magnetic, they attract other ideas to them.

Did you use any specific routines to help you write from day to day?

I tried to work on the book every day, without fail. I’d aim for a thousand words, but even I only managed one hundred it was better than nothing. I’d gone part time and taken a demotion in my NHS job in order to free myself up to write, so I told myself I wasn’t going to waste the extra days watching daytime TV or lying in bed.

How to write a best-selling debut novel: top tips from 'Before I Go To Sleep' author S J Watson