When SJ Watson brought out his debut novel Before I Go To Sleep in 2011, he experienced the stuff of dreams.

Not only did the book – about a woman with amnesia who wakes up each morning unable to remember the day before – become a runaway hit, selling more than four million copies in over 40 languages, it won the Crime Writers’ Association award for best debut novel, and last year made it to the big screen, in the movie adaptation starring Colin Firth, Nicole Kidman and Mark Strong.

Today, Steve John Watson – a balding, bespectacled, 44-year-old former British National Health Service (NHS) worker – admits he was worried about the level of expectation when he sat down to write book number two, Second Life.

“I wrote Before I Go To Sleep in a state of blissful ignorance. My biggest dream was that somebody else might like it, a literary agent and maybe
 a publisher. I never allowed myself to think I’d have more than a handful of readers.

“Writing Second Life has been a very different process, to know that I have 40 editors around the world who are interested, but more importantly, millions of readers. For a while, I was finding it difficult to find that space where I wasn’t trying to second-guess what people want. It took a while to find that quietness, and just write what I wanted to write.”

He writes in the first person as a woman in Second Life, whose protagonist Julia is a photographer and recovering alcoholic who lives with her kindly surgeon husband and their adopted son, who is her sister’s child. When her sister is murdered outside her flat in Paris, Julia sets out to find the killer, discovering that her sibling was using dating sites to meet men. She throws herself into her sister’s world to try to draw out the murderer, but ends up coaxed into experimentation of her own kind.

So, is internet dating something the bestselling author, who lives with Nick, his partner of 10 years, is familiar with? “In the past I’ve used dating sites, but I’m in a relationship now,” he says with a smile.

However, on one occasion, he recalls, he encountered a potentially scary scenario after going to the home of somebody he’d met online.

“I was with somebody I’d just met, and he locked the door and put the key in his pocket. At one point I became a little panicked, because 
I wanted to leave and he wanted me to stay. It was friendly from his point of view, but I was thinking, ‘This is a potentially risky situation’.”

The past four years have been a whirlwind for the West Midlands-born author who, until his writing success, worked with hearing-impaired children as an NHS audiologist.

At the time he started penning his novel he was 37, his boss was retiring, and he knew he was going to be offered a promotion to that job. 
But he didn’t want his life to go in that direction.

“I was nearly 40. When you’re young, there’s plenty of time. Then you get to an age where there isn’t. And I always thought that one day I would have a proper go as a writer. When I had my midlife crisis, I didn’t go and crash a motorbike or buy a Ferrari – I went on a writing course.”

He was accepted on the inaugural Writing A Novel course at the Faber Academy in 2009, and on the last day, he was taken on by a literary agent.

The idea for Before I Go To Sleep came from reading a lot about amnesia, discovering that it could be caused by physical or emotional trauma.

Watson decided to write under the initials SJ rather than his full name because it’s a woman’s story, and his editors didn’t want readers to know it was written by a man.

“Before it was even published there was a buzz about it. It felt like it could be big,” he recalls.

Since his debut, he has moved to a nicer flat in London – but he still hasn’t bought a Ferrari. He can afford to take more taxis though, he says.

“It was a whirlwind time, but I think I have my feet firmly on the ground, my friends and family see to that. If I ever showed any sign of developing a big head, they would be the first to say, ‘Oi, mate, don’t forget where you’re from’.”

He enjoyed the film adaptation of his first book, but wasn’t involved in the movie, he says, although he did go on set and meet the stars.

“They were very polite and nice. I was a little bit star-struck, like a kid in a sweet shop. The nearest I’d ever been to a filmset was watching the DVD extras – I was star-struck by the bloke carrying the camera wires and plugging stuff in!”

There is already interest from the film industry in his second book, and he admits he wrote Second Life with a more cinematic feel, featuring different locations which lend themselves to the big screen, but books have always been his first love.

Brought up in England’s Stourbridge, Watson’s father worked as a metallurgist at the local steel works, while his mother stayed at home. “I’m an only child. I could easily lose myself in a book. I wasn’t very sporty – so while the other kids were out playing football, I was indoors with a book.”

He was just starting a physics degree at Birmingham University when his parents split up.

“I was 18. In some ways, it wasn’t as much of a trauma as it may have been if I’d been younger. But their marriage hadn’t been happy for a while. It was a difficult time, but in a weird way, I’d grown up being very close to my mother – and still am.”

Now he’s made the big time in publishing, he’s signed up for two more books and already has the idea for the next one – but he’s not sharing it with me.

“I think of ideas in terms of a relationship. You flirt with an idea because you like the look of it, then you think, ‘I really like you’, so you start a relationship. Then you go through the stage of thinking, ‘I’m a bit bored with you now but we’re kind of committed’. Then another idea will tap you on the shoulder and go, ‘I’m quite attractive too’. And you gradually start to have an affair with your other idea. That’s been going on for a while.”