When Brian Conaghan asked Sarah Crossan to write a book with him, it took her about 30 seconds to say yes. The pair had never met - he lives in Dublin; she lives in Hertfordshire - but they admired each other's novels for young adults and embarked on what they describe as a "frenzied" collaboration. They finished the first draft of We Come Apart - a story of friendship and romance between two teenagers from very different backgrounds - in five-and-a-half weeks. "And I was away for a week of that," says Crossan.
The novel is written in free verse - non-rhyming poems - and alternates between the point of view of Nicu, a Roma boy, and Jess, a feisty British girl. The teenagers navigate their way through language barriers, bullying and racism until they come to a greater understanding of each other, and themselves.
Initially, Conaghan wrote the character of Nicu, and Crossan wrote Jess, but during the editing process they shared ownership of the voices.
And although they were engaged in "email tennis", sending each other chapters almost every day, they never discussed the plot.
"We didn't want to influence one another," says Crossan. "Hopefully the reader feels that, because they don't necessarily know where it's going to go next."
The writers didn't meet until after they'd finished the first draft, but their careers have followed similar trajectories. Crossan was born in Dublin and moved to the UK as a child. Conaghan grew up near Glasgow and has lived in Dublin - where his wife is from - since 2007. They were both teachers and both tried to write fiction for adults before they found success writing for older children. They share a common interest in outsiders, having both, at various stages, felt like outsiders themselves.
Over the past five years, their work has been widely acclaimed. Crossan won several prizes, including the 2016 Carnegie Medal, for One - her novel about 16-year-old conjoined twins - and Conaghan's third novel, The Bombs That Brought Us Together, won the 2016 Costa Children's Book Award.
They are used to tackling dark themes in their fiction and the characters in We Come Apart are at crisis point for good reason. Nicu's parents have arranged for him to marry a girl he's never met back home in Romania. Jess's stepfather beats and humiliates her mother, forcing Jess to record the violence on his mobile phone. Crossan says that as a mother (she has a four-year-old daughter) and an ex-teacher, she feels a sense of responsibility in writing for young people. "I want to say, 'Even though things aren't okay, it's going to be okay'... I'm not talking about censorship. I'm just talking about finding a way to make things palatable."
Some of the most disturbing scenes in We Come Apart are understated rather than explicit, the subtlety facilitated in part by the spaces and line breaks in the poetry.
Before Brexit, the writers had discussed how relevant the racism in the novel would be but following the vote, they no longer wondered. They wanted the depiction of Nicu to be as authentic as possible, and while they didn't have a sensitivity reader - someone who reads a manuscript for bias - they did consult a friend of Crossan's who is half Roma. Questionably perhaps, Nicu speaks in broken English. "Tata say we here for/short time/only/to make the Queen's cash..."
"I think the Nicu language is probably the most beautiful, colourful, creative language in the book," says Crossan.
"I'm sure from the other perspective somebody could say, 'It's an insult that he's not able to speak correctly,' but actually, from our perspective, he speaks beautifully."
At over 300 pages, We Come Apart is extraordinarily gripping and feels like a third of its length.
"What's great about the verse is you don't have to do the boring bits in between," says Crossan, whose other verse novels include One as well as her bestselling debut novel, The Weight of Water.
"You can do the snapshots. I think that's why people rush through it, because it's like looking at lots of photographs."
Conaghan thinks a teenager who might normally struggle with reading could get through 20 pages of We Come Apart relatively quickly. He always writes with his "teacher's hat on" and finds that his teaching experience in Dublin VECs and Glasgow's inner city has been useful for his writing.
"You pick up on the sounds of how kids are speaking. You pick up on the atmosphere and you pick up on some of the subtexts that are going on." He didn't necessarily intend his first YA (young adult) book to be for teenagers. Rather, he had an idea for a story set in a school and a series of characters began to emerge.
"I felt I could understand their voices," he says. "I think I knew them from myself, but I also knew them from teaching."
The book turned into When Mr Dog Bites, which features a 16-year-old boy with Tourette's syndrome. Conaghan's publishers decided it would suit the YA market - 13 and older - and since then, that's been his niche.
He loves the sense of possibility that comes with writing teenage characters; Crossan loves catching her characters at "those moments where they have to make moral decisions for themselves".
Both describe their collaboration on We Come Apart as uniquely invigorating and say the emotional support was invaluable, especially since writing is normally such an isolated process.
"Usually when I'm thinking about a project constantly, there's a big cloud hanging over me," says Conaghan, "but this time it was like a big rainbow."
Sarah and Brian are appearing as part of the Mountains to Sea Book Festival in County Hall, Dún Laoghaire, Dublin, today at 5.30pm www.mountainstosea.ie