Hepburn was born in Belgium in 1929, the child of a Christian Scientist Dutch baroness and a British-Irish 'financial advisor', Anthony Hepburn-Ruston, whose background remains shady despite Walker's sleuthing in dusty archives. The couple were great fans of Hitler. She became an active member of Mosley's British Union of Fascists; he, after abandoning wife and child when Audrey was six, ran the Nazis' London propaganda agency. Both paid for their dodgy judgement when the war came, he in a British internment camp, she, having switched sides, under German occupation in Holland, where she and the teenage Audrey were lucky to survive the German reprisals after the botched Allied offensive on Arnhem. After the war, Hepburn-Ruston disappeared, though, as Walker triumphantly reveals, Audrey did meet him once more, secretly, when he was an old man.
Walker believes that the need to keep her unwitting link with Fascism from the unforgiving American press - exposure might have ruined her career - accounted for Hepburn's exceptional self-control and caginess, and that the suffering she had witnessed in Holland stayed with her all her life. If he is right - certainly the interviews she gave were cautious and bland even by Hollywood standards - it casts interesting shadows across a professional persona whose very essence remained for years ingenuousness and a kind of tender frothiness.
Considering that she became the highest-paid star of her generation, Hepburn's talents were limited. She was a trained ballet dancer, but as an actress she relied on spontaneity rather than technique. What she did have were those stunning and remarkably photogenic looks - lithe, fragile, mischievous and, reassuringly for film-makers intent on family entertainment, curiously unerotic. After a brief starlet phase in London in the early Fifties (including a bit-part in The Lavender Hill Mob), she landed the jobs that were to cast her image in stone: the title role in the American stage version of Gigi ('Voila ma Gigi]' cried the famously picky Colette from her wheelchair on first spotting Hepburn) and the European princess who escapes from her ivory tower for a happy spree of Vespa-riding and ice cream-eating with Gregory Peck in William Wyler's Roman Holiday.
In a rare moment of purple breathlessness (another is when he describes her image as being 'as immutable as any Euclid proposition'), Alexander Walker sees this double break as proof that 'the gods were head over heels in love with Audrey Hepburn'. But he is as sharp about this film as about all the others: the freshness that came from shooting entirely on location and very fast; the way that Hepburn's cinched-waist outfits gave her the look of a Disney cartoon heroine; the British public's quaint belief that this piece of amiable cinematic fluff might provide insights into the real-life travails of Princess Margaret, then recovering from her romance with the out-of-bounds Peter Townsend.
Actors are supposed to chafe against type- casting. Hepburn embraced it, in a string of winsome ingenue and fairy-tale roles (including, disastrously, a wood sprite in the film of W H Hudson's Green Mansions) until The Nun's Story, directed by Fred Zinnemann, brought the chance to play a serious grown-up without having to take her clothes off. This should have been a watershed, but somehow Hepburn couldn't or wouldn't quite shake off her screen innocence. Even as the call-girl in Breakfast at Tiffany's she managed to strip away the character's essential element of tarnish (Truman Capote had written the part for Marilyn Monroe) and replace it with a kind of wistful purity. Her looks adapted perfectly to the Sixties, though, making her a plausible screen partner, visually at least, for hot newcomers like Peter O'Toole and Albert Finney.
Frock-dependency may have been another limiting factor. The couturier Givenchy was a lifelong friend and Hepburn wore his fabulously refined dresses, on screen and off, at every opportunity. She told Cecil Beaton, costume designer for My Fair Lady, that she didn't want to play Eliza because 'she doesn't have enough pretty clothes'. Better reasons might have been that she was not the stuff of which cockney flower-sellers are made and that, unlike Julie Andrews (to whom many thought the part rightly belonged), she couldn't sing very well. She was quietly crestfallen when, after she had recorded all Lerner and Loewe's songs, her voice was over-dubbed on the soundtrack by a professional singer.
But Hepburn was no fool, on or off the screen. Billy Wilder liked using her because 'she looks as if she could spell schizophrenia'. There were occasional displays of megastarriness - when filming The Nun's Story she insisted on having a bidet shipped out to Africa for her personal use - but for the most part she was almost disappointingly sensible: shrewd about money (hence the choice of Switzerland as her permanent home), a fond mother and keen on domestic bliss. The failure of her marriages, to the actor Mel Ferrer and an Italian psychiatrist, seems not to have been due to any lack of effort on her part. She spent her last years happily with Robert Wolders, the widower of Merle Oberon.
Alexander Walker's account is briskly detailed, sympathetic and entertaining, but it achieves resonance only at the end, at just the point where you might expect a slide into pure schmaltz. In 1988, when she was nearly 60, Hepburn became a 'goodwill ambassador' for Unicef. Any cynical assumption that this involved only occasional appearances at charity premieres is quickly dispelled. She travelled, uncomfortably and it seems compulsively, to Bangladesh, El Salvador, Guatemala, Kenya, Sudan and Somalia, to publicise the plight of the child victims of war and famine. A photograph taken in 1992 shows her looking almost as haggard as the starving African boy posing with her. Whatever her motivation - Walker invokes those haunting wartime memories and gratitude for her own survival - she brought to this phase of her life not just commitment, but a passion rarely evident in her acting work. It is this evidence of previously concealed depths which makes her premature death so moving: the onset of symptoms in Africa, the diagnosis of cancer of the colon, the unsuccessful operation in Los Angeles, the dignified return to Switzerland to die.
One of the mourners at her funeral was the French actor Alain Delon, who, arriving at the last minute, joked that he always had been late for his dates with Audrey. What dates? This is the first we've heard of them. It is faintly consoling to think that this determinedly private actress is still keeping a few of her secrets.