In her elegant New York apartment, surrounded by books and photographs of her beloved husband and daughter, Joan Didion has been struggling to understand why her tale of bereavement has enjoyed such literary acclaim and huge sales. After all, if it were just a matter of wallowing in sadness, The Year of Magical Thinking might well have had a short shelf life.

Finally, though, this tiny, elfin woman has concluded that she has written a love story. “People read it as a book about marriage, about the family - not about death per se,” she explains.

Didion’s husband, the novelist and screen-writer John Gregory Dunne, died suddenly of a heart attack in 2003 while their daughter, Quintana Roo, lay in a coma, suffering from pneumonia and septic shock. “You sit down to dinner and life as you know it ends,” Didion wrote with raw simplicity.

That was the book. In Didion’s one-woman play of the same name – directed by David Hare and opening at the National Theatre later this month – the actress who plays her then shifts her gaze to the audience. “The details will be different, but it will happen to you. That’s what I’m here to tell you,” she warns ominously.

The book was mostly about Dunne, Didion’s lifelong companion, but the play is as much about Quintana, who is remembered as a beautiful, carefree young girl with hair green from chlorine because of her love of swimming.

Didion had not meant to tell Quintana as soon as she awoke from her coma that her father had died – but she was overcome by an urge to be honest. Her daughter spent a further year and a half in and out of intensive care, by which time the book was already written. The play filled in her story. “It was a natural progression,” Didion says. “It was something I had to do. I had to write it down.”

After her husband’s death, she had made it her mission to keep Quintana alive, “to do for her what I failed to do for her father”, as she writes in the play. On one occasion she even put on blue hospital scrubs, desperate to butt in and take charge. In the event Quintana died at the age of 39; the child had passed away before the parent.

“I didn’t realise the enormity of that until long after it had happened,” says Didion, 73, who is regarded as a grande dame of American letters. Whenever she has to fill out a form or passes the New York hospital where Quintana died, it strikes her with renewed force that she has no dependants.

On trips to London with Dunne, Didion used to stay at the Dorchester – but she does not want to revisit old haunts when she arrives later this week to see the first British preview of the play. “It was the sort of place we used to stay when we were on movie money,” she says.

She speaks so softly that I find myself lowering my voice to match hers. Writing the book and play, she murmurs, allowed her to exercise a degree of control that had been lacking in real life.

“Writers choose a way of living that allows them to run their own show,” she says. “In all truth I had no idea how much I valued control until I was working on the play.” Originally she was asked if she wanted to play herself: “Unthinkable,” she says, shuddering. Her place has been filled by Vanessa Redgrave, a friend from the days when Didion and her husband were working in Hollywood.

Redgrave, 71, has already performed the role on Broadway. “I never thought of her as playing me,” Didion reflects, “and she never thought she was playing me, either. Obviously she’s taller than I am. But actually we’re not all that different.”

Redgrave may be far to the left but she has a “very clear-headed view of right and wrong”, Didion says. “She can’t understand why anyone would abide anyone doing wrong – and that leads her into ‘positions’.” Didion says she adores the entire Redgrave tribe of actors.

Her own family life has had interesting tensions. Her husband, brother of the Vanity Fair writer Dominick Dunne, was from an Irish family of brooding pessimists. “They always talked about their ‘curse’ and imagined that worse things happened to them than anybody else,” she recalls. “One brother committed suicide, and a family member was murdered. Terrible things did happen to John’s family . . . but it’s come to my attention that they happen to all families.”

Didion herself, the Californian descendant of pioneers who trekked across America, believes that the spirit of optimism is in her blood. “We don’t look at the negative side of life. We just keep moving west.”

It never occurred to her that her life would have a tragic denouement. “Denial does not begin to describe the state I was in,” she says. When Quintana was ill, “I was fine as long as I had a crisis to deal with. When the crisis wasn’t right there, filling the screen, I didn’t quite know what to do.”

Although her life before the events of the book was mostly charmed, Didion’s writing has often been edgy and bleak. When I tell her I have a 10-year-old daughter, she asks perceptively: “Are you going through that thing with your daughter where she is preparing for the separation of adolescence?” She then takes me aback by saying, “When Quintana was about that age, I wrote a novel, A Book of Common Prayer, about a woman whose daughter bombs the Transamerica building and goes underground. She loses her daughter. By the time I finished the novel two years later I realised the entire emotional impetus was my preparing for Quintana leaving.”

It is a violent image but her recollections of Quintana remain sweet and tender. She had come to her as a newborn baby after Didion happened to tell an obstetrician at a Christmas party that she was thinking of adopting. A few months later he phoned. “John took the call,” Didion recalls. “He got me out of the shower and said, ‘A baby’s been born. Do we want her?’ And I started to cry . . .” Three days later they brought her home from hospital. “She was a gift. Just a gift.”

In California the family had a glamorous life. Together Didion and Dunne wrote the screenplay for Up Close and Personal, starring Robert Redford and Michelle Pfeiffer, and Barbra Streisand’s A Star Is Born.

The Year of Magical Thinking is a recollection of a partnership that was loving and comfortable. On the day her husband died, Didion had brought him a scotch and was making a salad. “He was sitting across from me, talking . . . Then he wasn’t.” She thought he must be pretending to be dead.

Her husband had had premonitions of death that she’d brushed aside, telling him that he was merely depressed. Quintana too, she believes, knew that she would die.

Didion is fascinated by the literature of the Middle Ages, in which death often gives warning of its arrival – for example, Gawain, the knight of the Round Table, says: “I tell you that I shall not live two days.” “And didn’t he call that one right?” Didion says.

Reflecting on the deaths of her husband and daughter, she adds: “It’s the one thing I feel remiss about in both cases – I didn’t listen. I kept thinking: maybe if I’d listened, I could have affected the rest. But this is ridiculous.” She is not reconciled to her loss but feels blessed by the love they shared. “I do. I really do,” she stresses. That is the wonder of magical thinking.

The Year of Magical Thinking is at the National Theatre, London, from April 25; www.nationaltheatre.org.uk 020 7452 3000

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