Interview: A stage version of Joan Didion's painfully honest account of her husband's death comes to London

She may be the First Lady of modern American writing, but it has taken Joan Didion 70 years to figure out some basic truths about herself that will have seemed apparent for decades to readers from the clues that run through her books.

The fictional heroines, for example, who run so wildly out of control in novels like Play It As It Lays and A Book of Common Prayer – bad things happen to them that a buttoned-up "cool customer", as Didion was described by a hospital social worker at the time of her husband's death, might hope, by a triumph of pragmatism over naked emotion, to avoid. But this didn't work in real life. And, even while shedding a few frostproof layers in The Year of Magical Thinking, the memoir that describes Didion's attempts to "handle" the agony of bereavement, she failed to realise that she was a control-freak.

"It may amaze you that it took me working on the play, my first work for the theatre, to discover I had a controlling personality," she says wryly when we meet, still as frighteningly waif-like as described by previous interviewers: "so thin that if you touch her back you can feel the ribs, like ridges on a roll-top desk," according to The New York Times.

Didion has just recovered from a month of confinement to her sickbed, and here comes another insight – that finally getting to fulfil her fantasy of becoming a permanent recluse made her realise it was simply that: "I was just too bored – I realised I needed to be in the world after all."

Thus the formerly very brittle Didion, who has managed to share her troubled mental-health history through her non-fiction without revealing much of herself and, apparently, without garnering many insights, seems ready, at 71, to wear her heart a little closer to her elegant sleeve.

She talks frankly on the The South Bank Show about surviving the sudden death of her husband and collaborator of 40 years, the writer John Gregory Dunne, in 2003, and their only daughter, Quintana, who was seriously ill and in a coma at the time of the death, less than two years later.

Today in her Madison Avenue apartment – the couple fled to New York after years of chronicling California craziness from the inside and remaining, in Didion's own words "terrifically, terribly dependent on one another" – she is frank in discussing the ramifications of the death, which came a month before the couple's 40th wedding anniversary, on her writing, her sanity and her social life.

"The first piece I did after John died – a political piece – was really hard to do without anyone to read it," admits the strangely self-doubting author, whose husband had made her a gift of his approbation just a few weeks before he died:

" 'Don't ever tell me again you can't write,' John said to me as he closed the book. 'That's my birthday present to you,'" she recounts in her memoir.

Today she amplifies: "Usually John read things first, but once I started The Year of Magical Thinking I didn't feel the need of a reader, because the process of writing that book was like a conversation with John. I'd go over the pages at night and find myself actually talking to him."

Didion and Dunne were, with their talents for serious journalism as well as best-selling fiction, regarded as the golden couple of the American intelligentsia, though it is surprising how much store she set by his approval, considering it is she who got most of the accolades. This year alone, she has received the National Book Foundation's annual Medal for Distinguished Contribution to American Letters for "her distinctive blend of spare, elegant prose and fierce intelligence", as well as an award from the Writer's Guild of America.

While Play It As It Lays, her second novel, published in 1970, acquired cult status as well as hitting the charts, it was her essays that were making the name of Didion as a serious political commentator. Two collections, Slouching Towards Bethlehem, 1968, and 1979's The White Album, defined the madness of California in the 1960s and 1970s. They also revealed a little of the madness Didion feared lay inside herself, as she became overtaken by migraine, paranoia and disconnected thinking.

In her preface to Slouching Towards Bethlehem, she writes: "I went to San Francisco because I had not been able to work in some months, had been paralysed by the conviction that writing was an irrelevant act, that the world as I had understood it no longer existed."

In The White Album, as well as sharing her medical reports and her shock at a diagnosis of multiple sclerosis – "I had, at this, time, a sharp apprehension... of what it was like to open the door to the stranger and find that the stranger did indeed have the knife," Didion reflects how the Manson killings were, shockingly, not that unexpected in her circle, where paranoia had been building for years.

As an unlikely member of Hollywood's inner circle (she is a conservative who did not start writing screenplays until several years later), Didion found herself horribly connected with the event, beyond just living in the same neighbourhood where the Sharon Tate and Leno LaBianca murders took place. She and Roman Polanski were godparents to the same child, and she had met Tate socially.

Didion later became confidante to Manson gang member Linda Kasabian, V C who turned state witness – years later, Kasabian visited Didion in New York.

It is such unexpected bits of personal information that humanise the otherwise frighteningly intellectual Didion – the fact that she sat at Jim Morrison's feet during a Doors recording session admiring his black vinyl trousers, and that she played hostess to Janis Joplin.

Twenty years ago she confessed her fear at going to El Salvador for her acclaimed work on the country – "I've never been so scared" – and today she reveals she was too afraid to literally research the freeway routes for Play It As It Lays: "I always drove off the freeway until I had to drive to Long Beach, and I didn't want to drive through South Central after the riot; I drove clutching the steering wheel like this," she demonstrates, with a rare chuckle.

Didion's latest best-selling memoir, which won America's National Book Prize last year and is still attracting sackloads of mail from the bereaved, will become known to a wider British public when David Hare's production of her play, also called The Year of Magical Thinking, reaches the National Theatre in the spring.

It's not the same work – Didion declined to change the book, in which Quintana appears to finally be recovering from her illness, to reflect the fact she died shortly before publication: "The book was finished," she explained simply at the time. But Quintana's death at 39 has been addressed in the play, though it was not the same exercise in catharsis as the book, which Didion describes as "writing myself out of" the craziness that overtook her when Dunne suddenly dropped dead.

Her opening mantra, oft-repeated throughout the text: "Life changes. Life changes in an instant. You sit down to dinner and life as you know it ends," resonated with thousands of people who wrote to say they were fellow members of the crazy club Didion so strongly feels she entered at the moment of that first bereavement:

"It was all a surprise to me, what people found in it. Everyone feels a little bit in that situation that death makes other people uncomfortable so they want you to move on and get over it. And you're not going to for a period of time, which doesn't make people happy.

"John died in 2003, so I'm quite a ways on now in that process," she says, when questioned about a grieving experience so intense she could not even bear to give her husband's shoes away, "but I don't know it's even started yet with Quintana."

With Magical Thinking coming into the shops within weeks of her daughter's death, Didion subsumed her grief in a book tour.

One of the oddest things about the play is that the tiny, bird-like, closed-as-a-clam Didion is played by the Amazonian and passionate Vanessa Redgrave. To the writer, though, it's perfect casting. "I've felt always felt emotionally close to Vanessa," she explains. "We've known each other for a while because her former husband Tony Richardson was one of our best friends in Los Angeles, and that family stays very close. We watched Natasha grow up and she watched Quintana, who was three or four years younger, grow up. It was like one big, mixed up family; Natasha got married out of this apartment."

The lovely Upper East Side flat, with its pale, polished wooden floors, is itself an expression of the paradox that Didion continually presents – a plain-talking woman who loves fancy things like fine furniture and elegant clothes, an urbanite who revels in a large, countrified kitchen replete with copper pans, a tough cookie who keeps a huge black-and-white blow-up of her late daughter as a five-year-old child propped up against the large living-room window.

This love of nice things brought out knives: in her 1980 essay "Joan Didion: Only Disconnect", the feminist writer Barbara Grizzuti Harrison snarled: "When I am asked why I do not find Joan Didion appealing, I am tempted to answer – not entirely facetiously – that my charity does not naturally extend itself to someone whose lavender love seats match exactly the potted orchids on her mantel, who has porcelain elephant end tables, who has chosen to burden her daughter with the name Quintana Roo."

Nevertheless, this large, light cocoon feels authentic, a real home where Didion tries to follow a strict structure when bringing a new work to life: "In an ideal world, I start at 11, have lunch at some point, write till six or seven, stop work, return phone calls, start dinner and get a drink. Then I sit and mark up my pages, just like looking at rushes, and by the time I've marked them up the drink is finished and dinner is ready.

"If I'm working in that rhythm, I can work fast – the rhythm is all. I try not to go out, but things get in the way," she sighs. "I have to go to benefit dinners, a friend's birthday party."

She hesitates to turn down invitations because her late husband's words are still ringing in her ears: "You know those endless 'if anything ever happens to me' conversations which people have and I never listened to? The need to stay in the world, not to close myself off, was something that John always warned me about."

But working on her first play was painful: "This was working with people 12 hours a day, and it was one of the most exhausting experiences I've ever had, simply the exposure to people, though once I got past that, it was intensely interesting." Hare was patient, she says, with her constant script-tweaking and inclination to "try and do everything – even the sets".

Trying to be more social has proven exhausting, too – "I find my mind wandering at dinners, even more since John died" – but she feels strongly that it's vital to be out and about in order to inform her writing: "It was a good lesson, being too ill to go out for the past month: be careful what you wish for. I'm not a very lonely person, but I got bored."

Now the pain of shingles is subsiding – "they never tell you about shingles, like they never tell you about death" – she is back to work on both a new non-fiction book and a screenplay about the Vietnam navy doctor and humanitarian hero Tom Dooley: "There were several years in the Sixties and Seventies when, along with Albert Schweitzer and the Pope, he was the most admired man in the world.

"It's a movie I should have started a year ago, and I'm kind of making it up as I go along... I don't know how it's going to work out." For the same reason, she declines even to discuss the book: "I don't know yet how that's going to turn out, either, and I don't want to freeze it in my mind by talking about it."

Although she was a year late starting the screenplay, she has not felt haunted by the fact this is her first solo script after all the collaborations with Dunne, including Up Close And Personal, True Confessions and the 1970s version of A Star Is Born.

"We always worked separately on every picture – John would start it, or I would, and the other person would come in halfway through and pick up."

As we part, she says she is looking forward to the future "with great optimism". She does seem, at least, free of the overwhelming and almost unwatchable sadness captured in her by the camera; finally, one feels, Didion has determined to move forward and engage with that exhausting outside world.

'Joan Didion: The South Bank Show' will be broadcast at 11.10pm on Sunday on ITV