Interview: Helen Fitzgerald, author

GEORGIE and Kay are twins, but the girls couldn't be more different. While Georgie is prickly and problematic, Kay is all sweetness and light. But they are identical in one awful aspect: both inherited a genetic kidney disease that will kill them if they can't find a viable transplant organ soon. Their father, Will, is torn. The girls' mother absconded years ago and he is a kidney short of being able to help them both himself. What's a dad to do?

The Donor's spin on Sophie's Choice is very 21st century, but it's also vintage Helen Fitzgerald, whose novels are renowned for making you laugh out loud on one page and cringe at grisly goings-on moments later.

Over a companionable coffee at Edinburgh's Circus Caf, Fitzgerald, 44, fills me in on her own story. Trawling the internet has already told me that she's one of 13 children and grew up in a rowdy Australian household in Kilmore, Victoria. The family share a passion for black humour, she says: "My dad's very funny, and it was always quite brutal, quite sarcastic. That sense of humour was always with me. Glasgow is like that, too, and I've been living there for 20 years now."

She studied English and History at the University of Melbourne, and at 23 prepared to tour the world with her boyfriend. They broke up, but it seemed silly to waste the ticket, so she went anyway.

A trek in Nepal nearly ended in disaster, when she and her party were caught in a freak snowstorm in the Himalayas. Still unwilling to call it a day, she cashed in her plane ticket and made her way to London. "I was living in a squat in Bayswater with a bunch of Aussies. I went to London University to study for year, reading Commonwealth Literature. And then I fell in love with Sergio."

She met Sergio Casci in London at a party hosted by a Scot she'd bonded with during the snowstorm. Then, Casci was a trainee with the BBC, but today he's a full-time screenwriter - his new film The Caller just had its premire at the Edinburgh International Film Festival.

The BBC then sent him from Bristol back to his native Glasgow to work on Reporting Scotland. "We had a long-distance relationship for a long time, with me coming up on the train. I was really poor, and I probably shouldn't say this, but I wouldn't buy a ticket.

"I'd hide in the toilet. I never told Sergio, because he's very straight. I'd say, 'Meet me outside the station - I'll be the one running.' "

Though she had always wanted to write, Fitzgerald devised a more pragmatic master plan. "I knew I wanted (to have] kids when I was about 30, so I knew I wanted a job that would allow for that with maternity leave. My daughter's 14 and my son's 11. I hit it right on the money! I've always been very organised."

She was a project worker in an Edinburgh hostel for high-risk offenders: "When I was there it was mostly sex offenders. It was the worst job I've ever, ever had - it was terrifying … I got that job because I'd worked at an administrative job in a hostel for ex-offenders in London, while I was studying. Criminal justice really interested me."

On the back of her experience she was accepted to Glasgow University to qualify as a criminal justice social worker. "I applied to Melbourne University, as well, and got in. That was a bit of a dilemma. I'd have gone home, but it would have meant dragging my boyfriend, now husband, kicking and screaming. He doesn't want to live there."

She was a social worker for ten years, seven of them in the Gorbals and Govanhill, working in child protection. "The stories were awful and people hate you. It's very hard doing a job where everybody hates you … You've got incredible power going into these houses saying, 'Your-next-door neighbour saw bruises on your child and now we're here to have a look'. Also there were people who, because it's such a culture of dependency, would turn up in the office and say, 'I can't deal with her anymore', and just leave their child there."

She also worked in Barlinnie, where she again dealt with sex offenders. After a decade, sick and tired of hearing their excuses - and of being frightened all the time - she quit, and decided to try and make a go of writing full time.

You would think that someone scunnered with the justice system would write about anything but crime and criminals, and in fact, Fitzgerald did set off in a different direction at first.

"I would never watch Taggart or other crime shows, and I don't read crime books, even now. I'm not interested in it particularly. I would have had it all day and it was the last thing I needed.

"When I started I wrote screenplays, always romantic comedies or straight comedy. I got a lot of development money and felt I was getting somewhere, but for six years it was development hell. I had learned a lot. I read every book there was on plot and character and dialogue, and really studied.

"One screenplay, The West Highland Way, was shortlisted for a film scheme with the BBC and Scottish Screen, and I was told there was a one in ten chance it would get made. I did the treatment and decided if it didn't get to the next stage, I would give up and write a book, because that's 'real' writing, and it's all mine."

The movie stalled, and she was about to have her son, so Fitzgerald rewrote her screenplay as a novel, titling it Dead Lovely. "About halfway through writing it I found myself killing people, and I really enjoyed it. I thought, I'll keep going and see what happens, because this just feels right."

Since then she has published My Last Confession, The Devil's Staircase, Bloody Women, and in America, the young adult novel, Amelia O'Donohue is SO Not a Virgin.

She jokes that she and Sergio are forever hollering questions through the walls of their Glasgow home, such as: "What's the best way to dissolve a body?" Laughing, she says, "The conversations we have over dinner! Our kids tell us off, because we're asking what's the best way to get away with it? And joking about that all the time."

Is there anything too gruesome to touch? "I'm not into a lot of blood and gore and don't ever describe that, but probably not. If I hear something where I think, 'Oh my God that's too horrible' - that's exactly what I want to write about!

"The next book is about parents who accidentally kill their baby. It's based on my personal experience of travelling on long-haul flights with kids - the stress of that! Everybody hates you on the plane, and it's horrible. Also you have to put medication and liquids into those clear bottles, so I've got this great set-up with bottles that get mixed up."

Very matter of factly she adds: "You know, killing babies happens a lot, and it's awful, and it's interesting. My characters cover it up, because they've got other children, and for other reasons. I'm writing it from their point of view so you'll understand."

That empathy can be disquieting. While reading The Donor I had call to question the alignment of my moral compass. There's at least one murder that feels, well, the only word is "right". Do these sorts of concerns ever give her pause, as well? "I knew this was about a good person who is driven to extremes. Having worked in Barlinnie, and seeing guys who were murderers, I got to know them and to realise that's not all they are. The day before they killed someone, they weren't murderers. What drove them to that situation?

"My greatest fear, having been brought up Catholic, has always been of doing something really wrong. Even now. I don't think we're all capable of cold-blooded murder, but situations happen. Obviously most of us have barriers that keep us from getting into those situations, but I haven't lived through terrible times, I haven't been in a war, or been seriously threatened by anyone. What if I was? What would I do? I don't know."

Her favourite sections of The Donor to write explored Will's relationship with his daughters, for this is as much a novel about how we love our children, as it is "dilemma fiction".

"I've got two kids and I absolutely love them equally, but so differently. That's partly to do with one's a girl and one's a boy. Being from such a big family, I've always been interested in the fact that kids are always competing. I'm second to last and there are 20 years from top to bottom. We're quite close but we do still compete for our parents' attention, and so do my kids over ours."

Though she's not an avid reader within her genre, Fitzgerald is on record as saying that crime writers are the nicest, most generous people around. So they're not constantly plotting against one another? "They're very supportive of each other and fun and gentle. A lot of crime writers make a living out of it, so it's a job. We all go on Twitter at the same time, have a bit of a chat, then we get our 1,000, 2,000 words done, have another bit of a chat. If we need each other's help, we're here for each other.

"We don't feel in competition. Denise Mina would say this too: someone who likes her books would also read me and also read Karen Campbell and also read Steve Mosby. They're voracious. They don't choose just one author."

She writes fast, but says that's possible because her books are only 60,000 words long. "I get 50,000 words and I'm usually at the end point. My editing is usually about adding another 10,000. Everything I do, I do quickly. I have to know that I care about how the characters have ended up … If I don't feel quite emotional when I reach the last page, usually crying, I know it's not finished yet."

• The Donor is out now from Faber & Faber, priced 7.99 for paperback and e-book editions.