It is a strange undertaking to publish a book that you don't, in some ways, want people to read. Eighteen months after her husband's death, Natascha McElhone went back over the diaries she had written and addressed to him, and found an account of grief unfolding in real time: repetitive, raw, endlessly circular. "People said, why on earth would you want to share that? And in a perverse way I don't. What I want is for it to be available for someone to read if they need it."
She was filming in LA, pregnant with her third child, when her husband's friend Neil rang. Martin Kelly was 43, in his prime, what even before the eulogising could only be described as a "brilliant plastic surgeon". The couple had just had their 10th wedding anniversary and the baby was due in six months.
"Are you alone?" asked Neil.
"Well, -ish," said McElhone. "Fire away."
"They did everything they could."
The great template for writing about loss, CS Lewis's A Grief Observed, minutely charted the fluctuations in mood that make up the formal stages of grief; flashes of joy, followed by guilt, followed by the plunge once more into lethargy. In those months following her husband's death, lethargy wasn't an option for McElhone: she had two small boys and a pregnancy to look after, as well as the bureaucratic nightmare of disentangling her late husband's affairs. The word "authentic" is used almost exclusively these days to imply its opposite, but After You, a short, compelling book, is so unvarnished – "Not some actress, not a presented version" – that anything it loses in style is made up for in immediacy.
Two years after Kelly's death, McElhone is still occasionally ambushed by shock. She and the children divide their time between London and LA, where she is currently filming the US TV show Californication. She knows the triggers by now. "Sleeplessness is so dodgy because my thoughts – I have less control over them. They gallop away and pull you along, and you go on these bizarre night journeys that, thank God, when it's daylight, subside. The other night, I had just put the baby back to sleep and I remember thinking, again – and it's been two years, but I do remember thinking: this is just ridiculous."
Kelly was going through the front door of their London home when he died from heart failure. He was discovered by a friend, another doctor, with whom he'd been going for a drink and who tried, fruitlessly, to revive him. He had no will and a badly sold life-insurance policy, so in the months after his death it looked as if the only way McElhone would be able to access the money was to sue her own children. (It was locked in trust until they turned 18, useless, she says, since "they needed a roof over their heads now". A solicitor eventually sorted it out.)
She started writing to Kelly almost immediately, as a release and an effort to contain her grief out of sight of her children; Theo, the eldest at eight, had asked her not to cry in front of him. The early entries are full of the derangements of a mind in shock; one of the first things that crossed her mind after hearing the news was a hope that the windows in their London home were shut, so that nothing of Kelly's spirit could escape. Later, when she returned home, there was, she writes, such a palpable sense of him – the shirt on the door; the lingering smell – that "there is a period of time where I think, someone is still buzzing; there is a reverberation of them around you that you clasp, latch on to, in the hope that it will materialise into something more than a vibration. And of course it never does. There's that hope. It's very irrational. And you know it is. But it still gives a sense of comfort or relief." These are things she could record only in the knowledge that she was "writing to someone who's not around and you're not going to get a response".
McElhone had to fight the urge to go back and rewrite the diaries and to take out what, at close range, seemed to her to be boring details, but that in the event make the book. At the urging of her editor, she kept in verbatim the hurried to-do lists made in the aftermath of Kelly's death and which feature, along with "get a solicitor" and "call Wilts council", items such as "write letter to him, put in his coffin w cds, book, boys fav pokemon cards".
The question of how much her sons would remember of their father was one of the things that most worried her. McElhone didn't want him to be built into a godlike figure, nor removed from casual reference in daily life. She wondered whether she should radically change her parenting style. At first, she says, she tried to compensate for her husband's absence by mimicking his role in the family. "With Martin I always played the straight man and he was the clown. I thought, ugh, they can't just have straight man, that's so dull. I tried for a while to do clown and straight man, and they got really confused. So I just decided to stick to my role. I decided quite early on that I couldn't be a mother and a father. It was quite a relief."
She wondered, too, at how much of a disadvantage having one parent would put them. "One of the things that tortured me was that children learn humour from witnessing the interplay of adults, and since I'm on my own, I did feel that was going to be a huge loss." In the end, she hopes, character will make up some of the shortfall. "Theo is incredibly witty. And ironic. Lots of smirks and eyebrows raised, just in the way that Martin used to do. Somehow it's in there."
A more formal memoir would have included background on the couple's marriage, how they got together in their mid-20s after years of running into each other. There were the spooky coincidences that, after the fact, they read back as signs of their cosmic compatibility. (McElhone was living in an apartment block Kelly had just vacated; his telephone number was one digit different from hers, etc.) She had been single for a couple of years, had last known Kelly in his party animal phase and was surprised to hear he had knuckled down and become a doctor. For their first date, she made him sit through a performance by Mikhail Baryshnikov. "I thought, he's really interesting and he's calmed down, and he is doing all this amazing stuff. I had just assumed he was wrapped up in some New York top model. Literally two weeks after that I knew I was in deep."
Looking back, her husband would say, "I can't believe you didn't think we were going to spend our lives together that first night."
The thing she misses most, she says, is having a co-conspirator. She watches the children romping on the lawn sometimes and "there's that feeling of, there's no one to play with any more".
Friends offered to be with her for the birth of the baby, but it struck a "false note", says McElhone, and she decided that if her husband, who had been present for the first two births, could not be there, then she would rather go it alone. In the aftermath of labour, she wanted to be quiet and think of him, rather than make small talk with someone who was very much not him.
The baby, Rex, pulled them all together; McElhone's eldest son wrote a poem "about Martin's soul flying out of one window and coming through another into Rex's". She has tried to mimic their childish ability to live in the moment. "If there's anything good [that's come out of it] I think it's that I don't have any expectations. I had quite a lot, when I was younger. Living with very limited expectations is a much more immediate way of living. You really do just make the best of everything you have. I guess kids have that ability, they wait in joyful anticipation of something rather than that sense of entitlement. In a strange way, grief puts you back there, if you've lost everything. You just open up in a way that you haven't for years and years, to anything good that comes along; it's just so precious."
The gratitude extends to work, which became suddenly easier; after appearing in The Truman Show and Solaris with George Clooney, chasing A-list jobs became less important than securing a reliable income. "The decisions are made easier. The work serves our lifestyle and that's all it's about at the moment."
Otis, her seven-year-old, occasionally lobbies for a new dad. He has described what he wants: someone large (like his father), and blond, like him and his mother. He has some specific people in mind, at the top of the list Wayne Rooney and David Attenborough. His mother gently suggested that, lovely as he is, Attenborough might be out of her age range. Otis relaxed his criteria. "He said, as long as he's big and strong, I don't even care about the blond hair and blue eyes any more. It was so tragic. He'd obviously thought to himself, oh well, maybe I was too demanding." But two seconds later, she says, he was playing outside, "and it had evaporated".
Has she been on any dates?
"No, not at all. Time is a huge factor. I don't meet anyone. It's so complicated. Where would you even start? I'd have to meet someone who didn't know me or anything about me, just two people meeting. And quietly say, by the way, there's quite a few people at home. The strange thing is I feel like they're the best part of me; they're the best I have to offer. It's a bonus rather than a hindrance. But then it would take quite a big guy to realise that. Someone quite emancipated. Unselfish."
The most powerful parts of the book are those moments of transition when, albeit fleetingly, she feels things starting to lift. There was a point when McElhone realised, with shock and dismay, that she did not want to be celibate for the rest of her life. There was a moment when she started to rebel against the weight of her loss. Suddenly she wondered what she was doing, writing to her dead husband like this. "Does your spirit die," she asked, "if I don't keep blowing air into it." She began to understand that "my grief for you is also my love for you fighting for its last few breaths."
At the beginning, she wrote to Kelly as to someone who was still available to her; "the ultimate in long-distance relationships". The question she asks in the book is whether it is possible to let this go and still live a happy life. Does she believe that? "I do. I really do." There is a middle ground, she hopes, between hanging on in a way people describe as "unhealthy" and allowing him space in their life. The house in London is opposite a graveyard and she can see Kelly's grave from her window, which, she says, "feels absolutely right".
McElhone is still writing to him, every few months, although it is different now. She smiles. "I should probably stop quite soon. I'm not really writing to him any more; I'll say, 'I'm writing to you' but it's not 'you'. It's just a habit I got into. No. He's a memory."
• After You, by Natascha McElhone, is published by Viking at £12.99. To order a copy for £12.49 (including UK mainland p&p), go to guardian.co.uk/bookshop.