When he created Christine Lucas, the star and narrator of his best-selling novel Before I Go to Sleep, SJ Watson says, he knew exactly who she was and how she looked. "I had a very strong image of her," says the 43-year-old English author, known to his nearest and dearest simply as Steve.

In the film of his book, Watson's heroine is played by Nicole Kidman. Is this how he visualised her?

"I don't imagine my characters as actors," he says. "They come with their own visual language. But as soon as I saw her in the wig and costume, I thought, 'You are totally Christine.' She seems to inhabit the character, in a very good way. I can't imagine Christine looking any different now, which is kind of nice really."

Christine Lucas is an amnesiac. As a result of a trauma suffered at the age of 29, she wakes every morning with no memory of who she is, where she is, how she got there. As the day unfolds, she pieces fragments of information together to form a sketchy self-portrait. Then, when she goes to sleep, the memory banks are wiped clean. In the morning, she has to start all over again with only her husband, Ben, to guide her.

Before I Go to Sleep is a classic page-turner, but it touches on some pretty big issues – how is identity constructed; can stories help us reclaim lost memories; how much can we trust those we most depend upon? – but above all, it's a well-wrought and rather dark psychological thriller.

As it happens, Watson says, "I didn't realise it was a thriller until my agent told me. I don't think I really knew what it was. I just had a story I wanted to tell, and a character whose life I wanted to explore."

It is now part of publishing folklore that the forum in which he did so was the inaugural Faber Academy "Writing a novel" course (a spot in the local edition of the full Faber course will set you back $6500).  The world is littered with creative writing alumni who have gone on to set zero bestseller charts alight, but S.J. Watson has become the poster boy for this booming cottage industry. His debut novel has been published in 42 languages, with sales now nudging the 5 million mark.

That's likely to prompt either heartbreak or inspiration in budding writers – as is the revelation that he got the idea for the story just weeks before the course started. He had originally planned to take in a novel he'd been working on, and struggling with, for a very long time but came to the conclusion that he would learn more by going in with a blank slate.

Watson was working in the National Health Service at the time, in audiology ("I miss the people," he says now, "but not the work"), and shortly before the course started he stumbled across the obituary of the celebrated Henry Gustav Molaison, known in scientific literature as HM. As a result of a brain operation to treat epilepsy when he was 27, HM was rendered incapable of creating new memories, but could otherwise function normally in most other respects.

Shortly after reading this, Watson caught sight of his own face in the mirror and began to wonder what it would feel like to not recognise it. And so the idea for his novel began to take root.

He wrote his first chapter on a Monday, and went into the course two days later. That was 2009. By the following year, he had an agent and a manuscript that was being circulated among British publishers. There was some interest, he says, "but we hadn't signed a deal, when I got an email from someone at Scott Free" – filmmaker Ridley Scott's production company. Somehow, they had got hold of a copy of his book, still in Microsoft Word format, and liked it enough to snap up the screen rights.

"It was very gratifying for me that they were so passionate about it," says Watson. "That was before it was even published, let alone a bestseller."

The film that Rowan Joffe – the son of Oscar-winner Roland Joffe (The Mission, The Killing Fields) – has written and directed takes some liberties with Watson's story. For a start, his heroine is now 40, down from the 47 she is in the book (47 is, incidentally, Kidman's age in real life, though evidently it's not something she wants to trumpet on the screen). She now lives in a rather grand but isolated modernist house in the country rather than a cramped terrace in North London. And, most importantly, she no longer tries to recover her memories with the aid of a journal given to her in secret by her neuropsychiatrist, Dr Nash, who may be friend or foe; in the film, she records them with the aid of a video camera.

This last is an entirely logical shift and one of which Watson thoroughly approves. But it only came quite late in the piece, several drafts into the script, "and that's when it all started to fall into place, because having endless shots of a woman hunched over a desk scribbling in a diary while there's a voiceover isn't inherently very dramatic or interesting to watch. That's when I thought, 'Yeah, this could be really good'."

Watson says Joffe went out of his way to make him feel involved in the process, though he says, "I made the decision very early on not to get involved unless they tried to do something or suggest something that I really didn't approve of."

In any case,he says, "there was nothing that I even disagreed with Rowan about."

He was a frequent visitor to the set, which he imagines was interpreted as control-freakery by some of the crew, but really, he insists, "it was just that I was like a kid in Disneyland, enjoying the process and lapping it up."

In some respects, he was also learning to let it go. "The film for me has been a process of handing the book over to other people. Now I can step back and concentrate on other things."

Top of the list is his follow-up novel. Called Second Life, it is another psychological thriller, due out in Britain in February. He has only just finished it, and the process of writing it was, he says, harder than he'd anticipated. He wrote and scrapped various drafts, suffered frequent bouts of self-doubt and lost confidence, and above all found the idea of writing for his newly acquired readership something he had to grapple with and eventually banish from his mind.

"You might think it's a bit like riding a bike," he says. "Once you've been published, you're off and running now, you know what you're doing, it's easy.

"If anything, I think it's almost the opposite. Every book you write it's like you have to relearn how to write."

Christine Lucas, you suspect, would know exactly how he feels.

Before I Go to Sleep is now screening. S.J. Watson's novel of the same name is published in Australia by Text.