Your first novel, Before I Go to Sleep, was published in 2011 and became an international bestseller. Did you feel under pressure when you were writing your second?
It depends whether it’s a good day or a bad day! This is a difficult question because I don’t want to come across as ‘poor me’. When I wrote Before I Go to Sleep, I was in a blissful state of being disconnected from anybody else – I wrote it with the hope I could finish it, first of all, and then with the hope that somebody else might like it. But I was writing Second Life knowing that I had editors all over the world that were eager for it, and more importantly in many ways, readers who were going to read it. There was a sense of pressure in getting it right. But ultimately I realised I had to just write a book that I loved.

What’s Second Life about?
It’s about a woman who is happy, if perhaps slightly unfulfilled, in her life. But everything is turned upside down when her sister is brutally murdered in an apparently random attack. That’s really the trigger – an investigation into her sister’s death leaves her tumbling into an exploration of her own desires.

As the title suggests, there’s a lot about online identity in the book. How did that idea begin for you?
The origin for the book came a long time ago, when I was still working in the NHS, and I just began following this woman’s blog. She was virtually housebound through illness and used to blog several times a day. She would talk about her life and her friends and her situation in a lot of detail. As I read it, I began to feel a real connection with her, I almost felt as though she was a friend. So I began to think about how of course, this is a one-way friendship, I don’t really know her at all, I only know what she’s chosen to present on her blog; and over time I started to see how somebody might become obsessed by somebody else online, and project on to that person their own situation and their own beliefs about who they were.

We’re very easily drawn into these glimpses into other people’s lives these days, aren’t we?
After reading [the blog] for a while, I knew what part of London this person lived in, I knew that she had an allotment, for example, I knew what bus route she got when she went out, because she would mention all these things in passing. And I thought if somebody wanted to meet this person, supposedly accidentally, it would be quite easy to do. Not that I wanted to do it, obviously, but that was one of the things that fed into this book – this idea of how much of ourselves we can give away, because the internet has this illusion of anonymity, and yet actually, things that we post can be there for ever.

On the other hand, you’re a Twitter user, aren’t you?
I love it. I think I would like Twitter anyway, but the fact that I’m an author now gives me something to talk about as opposed to just what I had for breakfast. And it’s a really lovely way of connecting with readers and people who’ve enjoyed your work.

Before I Go to Sleep was about an amnesiac who has to rebuild her life every single day. It’s not hard to see why it might have fascinated people. But do you have a sense of why it had a particular appeal?
The bizarre thing is, I never saw it as a frightening book, but maybe that’s just something about me! People talk about not being able to sleep afterwards, ironically, and I think that’s something about how a lot of crime and thriller fiction - less so now, but certainly a few years ago - was about the serial killer wearing a ski mask who stalks the streets, and things like that, which are terrifying on one hand, but for most people quite remote from their everyday life. With Before I Go to Sleep, what I’m doing is bringing fear within the home. It’s about the danger from the people that you’re surrounded by. It’s about trust.

You’d worked for many years as an audiologist, writing fiction in your spare time but not really getting anywhere. What changed?
Throughout my whole career, there was this idea gnawing at the back of my head, that this is second best. Always what I really wanted to do was to be a writer, to be a novelist and create fiction. I was working with children in the cochlear implant programme at St Thomas’s hospital – but as is inevitable in many careers, as you get more senior, you actually spend less time doing the thing you intended to do, less and less time working with patients and families in the clinic, and more time sitting in awful meetings, talking about how to save money.

I was approaching 40, and I thought, towards the end of my life, I’d much rather be looking back and thinking, I had a really good go at writing a novel and it didn’t work, so I carried on with the health service, than look back on my life and think, well, I never really tried, did I?

So you started working part-time, but you also signed up to a creative writing course run by the publishing house Faber and Faber…
I’m not a believer in fate, but the week I handed in my notice, I saw an advertisement for the Faber Academy. And it just felt like very good timing – this might be what I need to mark the beginning of this new phase in my life. My big fear was that I would go part-time and then spend the two days a week that I’d created for myself watching daytime television.

What would you have done if Before I Go to Sleep hadn’t worked out?
I remember thinking that if this wasn’t the book that I managed to get published, I’d write another one and another one and another one; I remember thinking that I won’t stop until either I have a book published or somebody puts a gun to my head and says please stop writing now, it’s never going to happen.

For my whole life up until that point, I’d been a very safe person; I’d made sensible decisions in terms of career and life direction. It was quite liberating to go – you know what, fuck my career. I’m going to take a risk and do something stupid.

The ‘something stupid’ sold around the world and got made into a film starring Nicole Kidman and Colin Firth. Did you like the movie?
I think it’s great. I love all those sci-fi bells and whistles, special effects CGI thrillers, but I think this is a great example of something which is quite Hitchcockian. I like the way it’s a 90-minute thrill-ride, and I think there aren’t enough of those around.

In both your books, you’ve written from the point of view of women. Why’s that?
I’ve really just been particularly interested in women’s place in society and telling those stories from a woman’s perspective. I never saw it as a brave decision – I’ve lived my life surrounded by women and lots of my friends are women. If I’d written either of these two books from a male perspective, I would still had to have made an imaginative leap.

And finally, why the initials?
It was first suggested by my agent – partly because we wanted to send Before I Go to Sleep out and for it not to be instantly obvious whether I was male or female. But for me, it was something I wanted to do because I had this worry that I had written from the female perspective in the first person, and if people read it and thought well this is clearly a bloke, then the whole book wouldn’t work. So I thought if we do send it out with just my initials, if people aren’t certain whether I’m male or female, then that would be reassuring to me that I had at least got the voice mostly right.

And also I quite like the ambiguity. It’s ironic, because in many ways I wanted to use my initials so that whether I was male or female would become a non-issue, but it’s had the opposite effect!

Second Life by SJ Watson is out on 12 February (Doubleday £14.99). To order a copy for £11.99 go to bookshop.theguardian.com or call 0330 333 6846