Four years after his debut, Watson's follow-up, Second Life, is more ambitious, encompassing a larger cast and more locations as it takes in London, Paris and Berlin.
"What I've learnt is that no two books are the same as the process of writing each of them was quite different," says Watson. "With Before I Go To Sleep, I didn't plan it out to any great extent but with this book, I planned it out a lot more as it's more expansive with more subplots, which is something Before I Go To Sleep didn't really have. It had just four characters - one of whom doesn't appear until four-fifths of the way through - and it was a more claustrophobic book, which was deliberate."
While Before I Go To Sleep focused on the amnesiac Christine Lucas, who has to completely rebuild her sense of self upon waking every morning, Second Life delves into the murky environs of the internet - specifically, online dating.
Searching for answers after her younger sister is mysteriously murdered, lead character Julia Plummer embarks upon an ill-advised affair after joining a sexual contact site, only to soon find herself out of her depth.
"They definitely both touch on the theme of identity, how that can be manipulated by other influences, and how that can change over time as well," says Watson. "Before I Go To Sleep is about somebody who looks back on her life - as much as she can - and remembers what she was like as a younger person and goes 'how did I get here?' Second Life has that in common because it's about somebody who has had a second life, a previous existence that she has given up and moved on from to a new stage in her life."
A recovered alcoholic and drug addict, Julia attempts to create a new future for herself, only to be forced to come to terms with who she once was. "I suppose that's also what Christine does every day in Before I Go To Sleep, constructing a new identity from the few materials she has," says Watson.
"Whereas Julia is much more knowing and knows what she's doing. That was one of the things that interested me about the internet - how on one hand, online personas and social networks allow you to pretty much reinvent yourself but by the same token they also preserve things. It perhaps becomes harder to escape your past as things you might want to forget are preserved forever on the internet."
Having chosen to publish under his initials instead of his given name so readers would initially be unaware of his gender, Watson has been praised for writing convincingly from a female perspective.
"I suppose I've opened myself up to criticism in a way I wouldn't if I was female," he says.
"But then I read a funny comment from Gillian Flynn about Gone Girl, where for about 20 minutes she thought, 'Oh my God, I've killed feminism!' The question of whether a man can be a feminist is not one for me to answer. But I certainly consider myself to be pro-feminist and it would deeply upset me if I thought I was upsetting womankind."
Crucially, both Christine and Julia are essentially unreliable narrators. "Just because I'm writing in the first-person doesn't necessarily mean you should believe everything I'm writing about," says Watson.
"One of the things this book is about is the idea of fantasy. People fantasise about all kinds of things they would never discuss, let alone actually do in real life. I believe what goes on in the mind is really interesting and what I'm writing about here is somebody who allows the boundary between fantasy and reality to blur slightly."
But while insisting that he doesn't see her as a victim, Watson has been criticised for making Julia pay a high price for her sexual misadventures. "It was important to me that she always has agency," he says.
"When I was growing up, a lot of women in crime fiction existed purely to be beautiful and to then get stabbed. I didn't want to write about a woman to whom terrible things happen just because she was walking down an alleyway wearing a short skirt. I wanted to write about a woman who does what she does knowingly and willingly because she thinks she's in control, as it's really a book about the illusion of control."
Second Life (Text Publishing $37) is out now.