Author Emma Henderson talks about her debut novel, which has been nominated for a prestigious prize

Emma Henderson talks about her moving new novel Grace Williams Says It Loud, which has been shortlisted for the Orange Prize for Fiction.

For most first-time authors, it's not the story that's the problem: it's how to tell it - and whose voice to write it in. Find a convincing narrator, make him or her believable and, perhaps most importantly, fashion that person into somebody a reader wants to spend time with, and often the rest falls into place. But for the Orange Prize nominee Emma Henderson, that was far too easy. Her narrator, the eponymous Grace Williams of her debut novel Grace Williams Says It Loud, is a sensitive girl who becomes an impressively eloquent woman through the course of the book. But - and it's a big but - she can barely talk. Paragraphs and pages of dialogue pass without a word from this vivid character. And that's because, as we're told early in the book by an orderly at the Briar Mental Institute, Grace Williams is a "spastic and a mental defective".

"It was an experiment at first," says the 52-year-old Henderson of a conceit - when Grace can speak it is in sentences of two words only - that is far less frustrating than it sounds. "I'm probably admitting too much here but the whole writing of it was a case of fumbling in the dark, so I was imposing these rules to see if they made the book better. I did some research into child psychology and development and found that babies when they learn to speak start by naming just one object at a time. "Milk", "bread", "mummy", and so on. The moment they put two words together - "milk, now" - they are effectively forming sentences and making themselves understood. I wanted to see how far I could take that."

It's interesting that Henderson should say the writing of her book was a struggle. She admits that it has been "knocking around in her head" since her early 20s, and it comes as no surprise whatsoever to learn that Henderson, too, had an older sister, Clare, whose life was spent in institutional care and who had similar physical and mental handicaps. There is a heartbreaking line in the book uttered by Grace's sister Sarah, who says she has a brother and two sisters, but one of them "doesn't count". It's what Henderson used to think about her own sister. I wonder whether the book took so long to write because she felt a duty to tell the "right" story, to give her real sister the voice she never had.

"Looking back now, I suppose yes, I have given my sister a voice. But it's also an entirely invented voice if you see what I mean, because I was so keen that this was fiction. I wasn't interested in writing a memoir. So although Sarah bears lots of obvious similarities to me - not least the exact age gap - I'd like to think I haven't been as unpleasant as she was. But by the same token I don't know whether I got the same feelings of closure or redemption that Sarah experiences in the book. I would never say that I actually had a relationship with Clare in the way Sarah does with Grace."

A thinly veiled misery memoir this is not, then. Actually, Henderson admits, it's a conventional love story: at the mental hospital where she is treated so badly, Grace meets Daniel, who has epilepsy and has lost his arms in an accident. They become friends, confidants, and, eventually, lovers. He fulfils an important role beyond giving Grace a reason to have an optimistic outlook on a life otherwise filled with shocking abuse. He tells her mesmerising stories that take them both far beyond the confines of their quasi-prison. In that sense Grace Williams Says It Loud is as much a paean to the power of storytelling as it is a straightforward tale of survival.

"I must be honest, I only became aware of that idea towards the end of the book," she says. "But I can really see it now and it is very satisfying to hear that others can too. I wanted it to be really obvious that it was a part of Grace's personality to soak up stories - stories that everybody tells but which not all of us really listen to. That's something I feel very passionate about myself of course: the power of stories to transform. As soon as I had Daniel as a character there was no question in my head that I could be as wild as I liked imaginatively."

It's this that makes Grace Williams Says It Loud such a compelling book. Although so much that happens to her is sad, depressing, or simply downright outrageous, it's also an immensely readable and warm-hearted novel that refuses to make clear judgements on her unfortunate parents or even the mental hospital system that causes so much pain.

"When I wrote some of the more unpleasant scenes, I was worried that I was exaggerating - but the research I did suggested that was far from the case," she says. "It was a horrible can of worms to open, and I was completely shocked and horrified about what I learnt. But, a bit like the autobiographical elements, I really felt I had to put all that aside, so the book wasn't just me beating a drum. I know that I deal with some pretty emotive things, but it's genuinely not meant to be an issue-based book. I know I wrote this book and I had a sister who was in a similar situation but that doesn't mean I feel qualified to comment about what was right and wrong. It's as simple as that."

Which, if nothing else, is really refreshing. The book it most resembles on the Orange shortlist is Emma Donoghue's Room - and just as that novel isn't really about being locked away in a basement by a kidnapper for years, but a mother's love for her son, so Grace Williams Says It Loud is more a story of an exuberant life than an investigation into the case of an abused mental patient. Grace Williams is - in her head - eloquent and, well, graceful. She is neglected and abused, but it's not a particularly dark story. Grace, somehow, rises above it all. In fact, in places it's actually rather funny.

"And I do want people to identify with her," Henderson says. "I know I do. I want to try and make people see beyond the surface of things. I could have made it much sadder, I could have made it less sad. But most of all I wanted to make it believable."

So, if it's the book she's waited most of her life to write, what happens next for Henderson?

"Well, because I've come to this so late in life, I've always felt I was writing against the clock," she laughs. "Being nominated for the Orange Prize and dealing with the expectation of what comes next doesn't put any more pressure on me than I would have put on myself if I hadn't been published - there's still the desire to write the things I want to write before I die."

She permits herself a little giggle. "But being nominated for my first book is really wonderful. I don't know whether it's sunk in yet, and maybe that's just as well."