To put it simply: even the downs felt like ups. Her movies glitter with tantrums - it's as if her leading men were testing out such impossible beauty to check that it was alive, and finding out that it was; her bad moods put you in a good one. The Nun's Story could easily have sunk into earnestness, but Hepburn in a habit was more watchable than most other actresses in ballgown and pearls. Twittering and luminous as ever, she made a life in the Church look like a Roman holiday.
For a major star, Hepburn has a slender body of work to her credit. But there was hardly any build-up, or tailing-off: she sprang ready-made into the spotlight, and stayed that way. One look at this full- skirted sprite, tootling round Rome on a Vespa, and we knew that cinema had thrown up one of its originals. Some movie stars are the best of their kind; others belong to no kind at all, or rather create a new kind to which no one else has the key. Casablanca, say, is staffed almost entirely by members of the second category, and Hepburn fell gracefully into it too.
Some might see in her a vestige of the early silent actresses, and certainly the eyes would have gleamed at any time in the history of movies, or indeed the history of art; you half expected to see her picture in the Tutankhamun exhibition. No director could be blamed for letting his camera stare at eyes like that. Stanley Donen was the most unashamed: Funny Face is true to its title, with Hepburn caught peeking over the top of a book or under the brim of a hat. Fred Astaire, playing a photographer, under-exposes a print so that her eyes gaze out from pure whiteness; the reverse happens in Charade, where they goggle with fear, and everything around them is lost in shadow.
Even so, silent movies would never have been enough; imagine Audrey Hepburn losing her voice. Most elves squeak, and it was a neat idea of God's to make someone who looked so boyish sound so grown-up. The accent was throaty and well-bred, but hard to place - after all, she had a Dutch mother, an English father, and a life that went from Belgium to Switzerland via Hollywood. It was perfect for the movies of her time - the cosmopolitan grace of the Fifties and early Sixties, when light comedy could uproot its characters and plant them anywhere in the world, confident that they would bloom overnight. Charade is at once so silly and so civilised that it makes you weep - Cary Grant dining with Hepburn on the Seine, pouring out his charm and seeing if she would like to taste it: 'Oh, you should see your face.' 'What's the matter with it?' 'It's lovely.' Later on, she returns the complaint: 'You know what's wrong with you? . . . Nothing.'
This exchange of sublime pleasantries would be impossible now. We are lucky that Hepburn was flourishing at a time when Hollywood was beside itself with wicked thoughts, but couldn't do much about them; the result was a kind of light- headed erotic innocence that the camera adored. Hepburn was a sex kitten without the sex, and the idea of a nude scene would simply have made her laugh; besides, who wants to peel off clothes like hers? Eliza Doolittle was not really her type, it is true, but she was certainly her style: be honest, would you have wanted Julie Andrews to wear the Ascot hat? 'I'm not an actress,' Hepburn once said, and you believed her. It wasn't that she had attended ballet rather than drama school, but that she didn't feel the need to fake anything. She heightened the spirits of those around her, or made them behave with uncontrollable gallantry. When Audrey Hepburn walked into the movies, all heaven broke loose.
Anthony Lane is film critic of the 'New Yorker'