"I knew from the start I would write for teenagers, I knew my central character would be a 17-year-old girl. I wanted the story to have a fantastical element, though I didn't know what it would be. I love to read lots of different things, I love Margaret Atwood, I love Kate Atkinson, I love Isabel Allende, but there's another part of me that loves pop culture. Joss Whedon is a huge idol of mine, I love everything he's ever made, I've watched every episode of Buffy the Vampire Slayer." Spark features a cat called Buffy.
"I love geeky stuff, I love comic books made into movies. So will I write contemporary fiction, literary fiction? You just have to be who you are, and this is the stuff that spins my tyres. Give me a girl with a super power! This idea of treating teen fiction as the paddling pool before you're a proper grown-up and you're able to cope with real books, it really winds me up. Don't make me pick. I want it all."
Wanting, for Craw, is the inexplicable thing. This is what Spark is ultimately about - why we want what we want, how much freedom we have to choose who we are, what it is that drives us. When she tells me, "It's inexplicable", she's actually talking about her own desire to write.
Craw has been writing most of her life. "I remember being given my first lockable diary - I would have been 6 or 7 - and being ecstatic at the potential of it - feeling that urge to write something that would make it worth locking. When I was about 8 my parents gave me a little Blue Brother typewriter, and that was just momentous. I wore that thing to death, I went through so many ribbons.
"What I wrote most throughout my childhood and high school was plays, I'd badger my friends and my brother to act them out with me. It would always end in tears. Into my 20s and 30s I was writing hideous poetry or scripts. I even wrote a screenplay. But I really doubted I had the inner fortitude to write a book - I thought I wouldn't have the discipline or the stamina. It seemed too big, too hard."
She trained as a teacher, and taught English at Christchurch Girls' High and Rangi Ruru Girls' School, before leaving to have the first of her three daughters. "Another big factor for me in writing for teens was that I taught teenage girls for nearly 10 years, it's a species I felt I understood." She dabbled in amateur acting - "mostly amateur; I did audition for Shortland Street once" - though deep down she knew it wasn't going to give her the creative satisfaction she was looking for.
Then, shortly after her youngest daughter was born, something shifted. She found she needed to write. "That sense of having to write something or I'd go mad - I had it on a really epic scale. I don't know why it was then. It's inexplicable. In my state of desperation, I was sitting on my bed one night doodling away in my journal, trying to come up with ideas, and I actually prayed this very desperate prayer, for God to give me an idea. I said, 'Right, here I am, I want a dream, I need an idea, I'm waiting, let's do this thing.' I went to sleep and that night I had the dream."
The story she developed to explain the dream - "why was I so fast, where did I get these reflexes, why was the person out in the forest my responsibility?" - took genetic engineering as its jumping-off point. Evie, her heroine - also the name of her youngest daughter - is a genetically enhanced bodyguard, programmed by her genes to feel an overmastering need to protect anyone with a certain genetic marker. She's superhuman, but she is also something very close to a slave.
"One of the motivating aspects of that premise for me is the enormous sense of injustice it carries with it. I'm probably quite justice-driven in terms of the things I react to. That thought of being bound to something, having your choices taken from you. It's such a creepy concept. It raises all those questions of personal responsibility - am I advocating people doing whatever the hell they like because they couldn't help it? No. But I love all those questions of nature versus nurture."
These are questions with sharp teeth for Craw, who was adopted, and met her birth mother only a couple of years before starting work on Spark. "When I met her for the first time I was gobsmacked by the power of DNA. Not just the obvious things - we looked alike, sure - but that I could be so similar to someone I had never spent a moment with. Even our mannerisms. It was a bit spooky. That got my creative juices going a bit. How much of who we're going to become is built into us at a cellular level?"
Spark (Walker Books $21.99) is out now.
