It wasn't until she went to Paris on holiday last year and was wandering by the Seine, free from domesticity, that she realised she had a story. Those bits became Landscape With Solitary Figure, an atmospheric portrait of a sensitive woman, and the man who takes interest in her greatest fear.
It's an unsettling novel. The lone figure of the title speaks of entertaining the occasional guest at her charming bungalow but we're not immediately sure of the significance.
Eventually we learn about the awful birthday party, and the terrible dinner, attended by snobs and drunks. What happens at these events and why does she feel so unsafe? All is revealed but in languorous good time, and with Koea's signature dark wit.
We're chatting in the author's Bayswater home, which is beautiful and tidy, yet has a wildness about it, bursting with paintings and antiques.
Later, we'll wander through her garden of clivias, which her surgeon son Jonathan recently pointed out were getting a bit out-of-hand.
Koea has made egg sandwiches, which she presents on dainty china plates. A charming, homely home is important not just to the writer but to her protagonist, Ellis Leigh. Ellis is also a writer with a garden of clivias. She too lives a solitary existence, not far from the sea. Ellis "just arrived" in Koea's mind, on one of the five days a week she sits at her keyboard, writing, "even when I'm sick". She is superstitious about the process, hence it had to be Ellis, not Alice. She pinched Ellis' surname from a ukulele teacher who writes to her from Taihape.
"If she'd spelt it Lee," she says, "I wouldn't have been interested."
In the evenings Koea will check what she's written, tinker with it and, if she's lucky, come up with an idea of what to write in the morning. She does this relentlessly, until a novel is formed.
"I never know what my books are about really, until I'm well into them. I knew a writer once - I didn't like her much - and she used to have a big plan on the wall and cards with different characters. I never had any of that. Maybe there are just different ways. I just think that's rather dead. You'd be so busy looking at cards or your plan, you wouldn't see something."
The naughtiness crept in because Landscape evolved over such a long period she'd not read which "bits" came before. When she finally put it together, she was surprised to find some was written in first person, some in third. No matter. Koea may have broken the rules - being an incorrect sort of writer - but the shifts in perspective paint a broader picture of who Ellis Leigh really is. At times she's an unreliable narrator. The abuse she suffers is so insidious, the victim so vulnerable, it's unsettling to see what the tormentor gets away with, or if, at times, it's even real.
"I wasn't sure either," says Koea. "The police say this about accidents: if there's an accident and several people have seen it, everybody's story is slightly different. They're all telling the truth as far as they're concerned but it's not the same. So there's this doubt there."
Why is the bully in the novel the way he is?
"I don't know. I wondered if he might've had Asperger's."
Perhaps this is what happens when you write as she does. The characters become entities unto themselves, the words are something to be captured on the day they decide to show up.
"You just have to write like your blood. Or one of your bones. You have to write like a bone lying on the page."
Koea has always been careful to distance herself from her characters. If they were in any way autobiographical, she says, she would have lived in India for 26 years as a maharajah's mistress (Sing To Me, Dreamer) - "How super" - or stabbed someone over lunch (The Lonely Margins Of The Sea). She is used to others blurring the lines. But anyone who has read Koea's kitchen memoirs, The Kindness of Strangers, will know there are threads of her life throughout her novels.
"You write about the sort of things you know but I think I've got a very isolated view because I'm inclined to be isolated, always have been."
She spent much of her childhood hiding from a violent and callous father, and later developed a stutter which caused her to withdraw socially. In 1987 her husband, George, died unexpectedly. In the memoir she touches briefly on a time when a good-looking but dull man invited her to dinner, a date that turned threatening, much like the relationship in Landscape. Many people, it seems, are not to be trusted.
"I must be an introvert, but it might have begun when I was just a little girl and I didn't speak and I was left out of things so an affliction became a lifelong modus. I don't like too much company.
"I don't think I'd be able to write a novel about somebody very sociable, or someone who thought a lot about motor cars or having her hair done, because I don't know about that.
I mainly have a lifelong habit of mooching about so I write about mooching about."
If you lived in Koea's home you'd have a grand time doing just that. "Oh, all these things, they're not very fashionable, never were really."
She loves being here on her own with her cats, Maisky and Mr Grey, her Vogue magazines, the occasional Sky documentary.
Parties aren't her thing but people must be, because she's an easy conversationalist. She laughs when she recounts a recent literary event she attended. During a rather long poetry reading she absentmindedly sidled over to the food table and bit down on a cracker, a loud crunch echoing through the hushed room.
"In a gentle but innocent and loopy kind of way I didn't think I had to do what everyone else did. I thought I was quite separate from everything. I was worried I seemed rude by biting into the cracker but it's interesting I noticed it of myself."
Landscape with Solitary Figure (Vintage $29.99) is out now. Shonagh Koea will appear at the Auckland Writers Festival on May 16.
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