Katharine Hepburn was the star of my very favourite film, The Philadelphia Story. She was also the star of my second favourite film (Adam's Rib), and my third (Woman of the Year), and she appeared in a large proportion of the other movies I can stand to watch without throwing something at the screen or falling asleep. The sheer scarcity, in cinema, of women who in any way resemble those unusual creatures we meet every day (our mothers, sisters, wives, lovers, daughters) has only intensified in the 20 years since Katharine Hepburn ceased making movies, and this has served to make her legacy more precious as time has passed.

From the earliest age I was devoted to her. My teenage bedroom, for many years a shrine to the Golden Age of Hollywood, reserved a whole half-wall for her alone. Amid the pictures of Cary Grant, Jimmy Stewart, Donald O'Connor, Ava Gardner, and the rest, Ms Hepburn - imperious, regal and red-headed (although this last was often disguised in the publicity shots) - sat high up by the cornice of the ceiling, like a madonna looking over the lesser saints. As a child I spent too much time worrying over her health, and wanting assurance from my father (also a fan and only 18 years her junior), that she would outlive us all. When she sailed through her late 80s without incident, I do recall becoming partially convinced of her immortality.

Possibly because she got to me so young, her effect is rather out of proportion with what any movie star should mean to anyone, but I am immensely grateful for it. The kind of woman she played, the kind of woman she was, is still the kind of woman I should like to be, and an incidental line of hers, from the aforementioned The Philadelphia Story, remains my lodestar every time I pick up a pen to write anything all: "The time to make your mind up about people is never!" This line was written by Donald Ogden Stewart, but in its utterly humanist commitment to the peculiarity and beauty of individuals, it was 100% Hepburn.

In that film the question at hand is class; Hepburn's Tracy Lord is trying to convince a class-conscious Jimmy Stewart that virtue is not restricted to the working men of the world, any more than honour rests solely with the rich. It is Tracy Lord's purpose to go to work on the cliches and generalisations of class, and it was Hepburn's unique position in Hollywood to chip away at some of America's more banal and oppressive received ideas. Whenever Hollywood thought it knew what a woman was, or what a black man was, what an intellectual might be, or what "sexiness" amounted to, well, Hepburn made a movie to turn the common thinking on its head, offering something irreducibly singular. Sometimes they liked it, but more often than not - especially in the early days - they didn't. It was another trait of Hepburn's never to give an inch. When David O Selznick told her she couldn't have the role of Scarlett O'Hara because he "couldn't see Rhett Butler chasing you for 10 years", she told him snootily that "some people's idea of sex appeal is different from yours", and stormed out of his office. It was never a question of Hepburn changing to suit Hollywood; Hollywood had to change to suit Hepburn.

Her bull-headedness was due to her east-coast upbringing: Protestant, hard-working, sporty, intellectual, liberal and somewhat severe. Cold showers were a staple of her childhood. Hepburn said they "gave me the impression that the bitterer the medicine, the better it was for you," and this strikes us as absolutely commensurate with her image on the big screen; never indulgent, always somehow utilitarian; only doing and using what was necessary. Ava Gardner you see in a big tub of bubbles, Hepburn in the Connecticut cold, standing in a bucket of ice water. Attributing to her childhood all her positive virtues, Hepburn always looked to her parents' lives and relationship as the model for her own. Her mother, Katharine Martha Houghton, known as "Kit", was a committed feminist, and an early graduate of Bryn Mawr college, one of the first institutions to offer women a PhD. She was good friends with Mrs Pankhurst, became the president of the Connecticut Women's Suffrage Association and in later years a vocal supporter of planned parenthood, despite giving birth to three boys and three girls. Her husband, Dr Thomas Norval Hepburn, could trace his ancestry back to James Hepburn, Earl of Bothwell, and third husband of Mary, Queen of Scots (who Hepburn played in 1936, rather lumpenly. Later she recognised that fiery Elizabeth would have been more her bag). From him, Hepburn got her hair and her family nickname, "Redtop", a great enthusiasm for all things physical and absolutely no understanding of feminine restriction.

Dr Hepburn made few distinctions between his sons and daughters. All of them played touch football, learned to wrestle, swim and sail, and were encouraged in the idea that intellect and action are two sides of the same coin, for either sex. Her father was the kind of man Hepburn most admired: "There are men of action and men of thought, and if you ever get a combination of the two, well, that's the top - you've got someone like dad."

Born in 1907, two years after her parents' first child, Tom, Hepburn grew up as a very jolly, tree-climbing, trouser-wearing, straight-talking tomboy, devoted to her older brother, and awkward with people outside her family circle. When she was 12 a tragedy occured, one that changed her life and seems to have gone some way to forming the actress she became. During a trip to New York, Katharine and Tom went to see the play A Conneticut Yankee in King Arthur's Court, in which there is a scene of a hanging. The next morning, when Katharine went into her brother's room to wake him, she found him hanging from the rafters by a bedsheet, already dead five hours. He was 15.

There was a history of suicide on both sides of the family, but her father believed it a stunt gone wrong. Either way, Hepburn was deeply affected by it. She began to attempt to take on many of her brother's traits, and in some way to replace him; she spoke of taking up medical studies at Yale, as he had intended to, and became involved in the sports he liked - golf and tennis and diving. Having no real academic ability, she never took that Yale place, but scraped through her exams sufficiently to follow her mother to Bryn Mawr. This college, often ridiculed for its supposed snobbery and blue-stocking atmosphere, was where Hepburn began to act, and also - or so her later critics complained - where she picked up that unbelievable accent, that "Bryn Mawr twang", with its peculiar, long English vowels and the sense that one is being spoken to from a pinnacle of absolute high-Yankee condescension.

Her class and ambivalent "femininity", were to become central to Hepburn's screen persona, and were also the qualities that made her "box office poison" for the best part of a decade. Selznick's reticence to let her play Scarlett referred pretty obviously to her body and we should begin with that. Her great love, Spencer Tracy, put it this way: "There ain't much meat on her, but what there is, is choice." And so it was. Slender without being remotely skinny, Hepburn was pretty much one long muscle, devoid of bust, but surprisingly shapely if seen from the back. She could work a dress like any Hollywood starlet, but your heart stopped to see her in a pair of wide slacks and crisp, white shirt. Her face was feline without being flirty, her cheekbones sepulchral but her lips full and generous. Her eyes - and there isn't a movie star who doesn't come down to the eyes in the end - had that knack of looking intelligently and passionately into the middle distance, a gaze that presidents strive for and occasionally attain. Her nose was more problematic. It struck some people as noble and full of sprightly character, but for a great deal of others it was rather too refined, hoydenish and superior. There are certain of her early films where a good 70% of the acting is coming from the nose down, and the average 1930s depression-era movie-goer was not in the mood to be looked down upon through quite so straight and severe an instrument. They didn't like her much as an aristocratic aviatrix in Christopher Strong (1933), and they liked her even less as an illiterate mountain girl in Spitfire ( 1934 ), which is something like watching Helena Bonham Carter play Cat in EastEnders.

But to really make them hate you, it is so important to spend an entire movie dressed as a boy and making Brian Aherne fall in love with you - while you are still dressed as a boy - and then have him say things like, "I don't know what it is that gives me a queer feeling when I look at you," as Hepburn did in the truly mesmeric transvestite comedy flop, Sylvia Scarlett ( 1935). The Shakespearean references were pretty much lost on her American audience, who were rather too busy worrying about where they were going to get their next meal and unwilling to allow much brain space to the homoerotic possibilities of Katharine Hepburn dressed in green suede.

Time magazine took the opportunity to point out, "Sylvia Scarlett reveals the interesting fact that Katharine Hepburn is better looking as a boy than a woman." There were hits during the 1930s, though, most notably Little Women, in which Hepburn played the greatest, most empathic and beautiful Jo March there ever has been or ever will be. But she was only playing good roles in hit movies - she could not yet carry a movie alone. Hepburn didn't help herself, either, with her on-set behavior, which was noted and commented upon by the usual LA gossip columnists sent by the magazines to investigate the potential starlet. They had been spun a red-haired, east-coast, high-society goddess by the studios and so were somewhat surprised to find a makeup-free woman striding around between takes in a pair of dungarees. The RKO publicity department asked her to stop wearing them. She refused. The next day, when she found them vanished from her dressing-room she walked around set in her knickers until they were returned to her. On another occasion, when denying to reporters that she was married (she was, but very briefly, to Ludlow Ogden Smith, a man she met at a college dance) and asked whether she had children she replied: "Yes, two white and three coloured."

It was around this time that Hepburn decided to return to the stage in a play called The Lakes and found herself on the receiving end of Dorothy Parker's poisonous little put-down: "Katharine Hepburn ran the gamut of emotion from A to B." In its way, this comment is true - Hepburn could not be stretched far beyond herself. But her triumph, like all the Golden Age actors, was to figure out that screen acting, in opposition to stage acting, hasn't got a damn thing to do with range. The present-day enthusiasm for actors who can play anything from the severely disabled to the heroic to the romantic and so on, with their multiple accents and tedious gurning - this all meant nothing to Bogart or Grant or Stewart nor, in the end, to Hepburn. It was by learning to play herself, and by continuing to do so, more or less, for the rest of her career, that Hepburn became a screen icon and a goddess.

Of The Philadelphia Story, Life magazine wrote, "When Katharine Hepburn sets out to play Katharine Hepburn, she is a sight to behold. Nobody is her equal," and I write now that if there is a greater pleasure in this world than watching her drunkenly singing Somewhere Over The Rainbow (she was many things, that girl, but she was no singer ) while wearing a dressing gown and being carried by Jimmy Stewart, well, I don't know it. There is another line in that movie that seems key to Hepburn. George Kittridge (John Howard), her soon-to-be-jilted fiance, complains that, "A husband expects his wife to behave herself, naturally." To which CK Dexter-Haven (Cary Grant), the ex-husband, offers a pointed correction: "To behave herself naturally." In this vanished comma and subsequent seismic shift of emphasis, we find the womanly wonder of Hepburn's 1940's comedies.

She was 36 when she made her first comedy with Spencer Tracy and to see her in Woman of the Year (1942) is to put a tongue to that great lie of contemporary culture, that tedious fascination with women between the ages of 16 and 25. To say she is in her prime doesn't even begin to cover it. She is a woman behaving herself naturally, without fear, without shame, and with the full confidence of her abilities. The battle and paradox of those Tracy/Hepburn vehicles is the same one they had to deal with in life: how to tame a great passion without either party submitting entirely to the other. This makes Woman of the Year (1942), Adam's Rib (1949)and Pat and Mike ( 1952) sound like dry stuff - nothing could be less true. Adam's Rib is simultaneously so funny and so sharp - and I mean, to the bone - on the subject of the gender war, that I have watched it with two different boyfriends and it has served twice to spark the kind of row that sends us into separate bedrooms and a good 12 hours of the silent treatment. The question of competition in a marriage of equals is so accurately skewered you feel yourself writhing on the pin.

Hepburn and Tracy play two lawyers, who find themselves defending and prosecuting against each other on the same case. One evening, after a long day in court, a supposedly friendly smack on Hepburn's backside from Tracy (he is giving her a massage) results in this unbeatable snatch of dialogue:

Tracy: What'you don't want your rub now? What are you - sore about a little slap?

Hepburn: You meant that, didn't you?

Tracy: Why, no ...

Hepburn: Yes you did, yes, I can tell a ... a slap from a slug!

Tracy: Well, OK ... OK ...

Hepburn: No, I'm not so sure it is, I'm not so sure I care to ... expose myself to typical, instinctive, masculine aggression!

Tracy: Oh, calm down ...

Hepburn: And it felt not only as if you meant it but as if you felt you had a right to! I can tell!

Tracy: What've you got back there? Radar equipment?

Oh, just go out and rent it. Although I am particularly susceptible to Kate in the 1940s, every decade of her career throws up humbling performances. She held the record for Oscar nominations until this year when Meryl pipped her, and any Hepburn devotee finds a place in their hearts for the grand guignol of Suddenly Last Summer (1959), despite the fact that she despised both the film and her treatment on the set - she spat in the director's face on the last day of filming. Also, the dynamite partnership with Humphrey Bogart in The African Queen (1951) is the equal of anything she did with Tracy.

In Bogart, she found the action-man dimension of her father that she loved, and he found a woman with similar chutzpah to his own wife, Bacall, who followed them to the difficult, insect-ridden location, reportedly a little nervous of how good a match her husband and his co-star looked on paper. She needn't have worried: Hepburn's romantic dedication to Tracy is now legendary, and I remember, again, as a child, hoping for the sort of relationship that seemed symbolised in the fact that even as he lay dying, Hepburn spent every day with him, lying on the floor beside his bed.

They never married because he was already married, and as a Catholic and family man, he never divorced. One can only feel horror and pity for the kind of wife who had to, before the whole world, recognise the shimmering immortality of that adulterous relationship. Tracy claimed "my wife and Kate like things just the way they are," and the truth or otherwise of this remained between the three of them, never publicly discussed. Tracy and Hepburn's last film together, Guess Who's Coming To Dinner (1967), was the first Hepburn movie I ever saw, aged about five, with a running commentary from my mother on the physical perfection of Sidney Poitier. The movie is sentimental, but the sentiment, both political and personal, is real - I struggle to think of another film of which this is so true.

Tracy, a long-term alcoholic, was basically dying during filming and when he delivers the last line, "If what they feel for each other is even half what we felt, then that is everything", Hepburn cries real tears. Six months later he was dead. They were both nominated for Oscars, and when she heard she had won again (Hepburn was not at the ceremony; she never picked up any of her four Oscars) her first and only question was "Did Spence get one too?" He did not, but she considered it a shared award.

So physical in her youth, always determined to peform her own stunts, old age struck Hepburn as one hell of a bore. She never hid like a startlet, or felt destroyed by lost looks (she never really lost her looks), but she was frequently fustrated by not being able to do what had once come so easily. She once cried with fustration at having to employ a 24-year-old stunt double to ride a bike for her in a film. At the time Hepburn herself was 72. Two days ago she died, aged 96. I don't know why I should be surprised, but I was, and when I found out, I wept, and felt ridiculous for weeping. How can someone you have never met make you cry? Two years ago I went to see The Philadelphia Story play on a big screen in Manhattan's Bryant Park. It was July and so hot my brother and I had been spending the day in the penguin exhibit in Central Park (we had no air conditioning), but then we heard about the film - our favourite film - playing outdoors, and rushed downtown.

We were too late to get a seat. It was packed like I have never seen any New York open space since the Dalai Lama came to Central Park. We were disconsolately looking for a wall to sit on, when suddenly two unholy fools, two morons, changed their minds and gave up their second-row seats. Hard to describe how happy we were. And then over the loudspeakers came some news: Hepburn had been taken ill in the night - gasps, I mean, real gasps - but it was OK - happy sighs - she was back from the hospital and wished us all well. We roared! And then the film started, and I said all the lines before they came and my brother asked me to shut the hell up. But I wasn't the only one at it. When Katharine whispered to Jimmy Stewart, "Put me in your pocket, Mike!", 1,000 people whispered with her. That was the best night at the movies I've ever had.

My teenage life was periodically dotted by melancholy little funerals, attended by me alone. I held one for Fred Astaire, and one for Gene Kelly and - most upsettingly, for he was the only real rival to Hepburn in my heart - Jimmy Stewart, who I was certain would survive long enough for me to meet him.

On these occasions I would light candles in my room, cry a bit and mark the photo on the wall with a little cross in the right-hand corner. I'm too old now to be quite the obsessive weirdo I was when I was 15, but I am still sufficiently blessed with leisure time to spend the next few weeks watching every Hepburn movie and documentary that graces the television screen. I strongly recommend you watch as many as you can manage. She is the last of the great stars, the very last, and my God, I will miss the thrilling feeling of re-watching Adam's Rib and knowing her to be still on the planet, still in that East 49th Street brownstone, still fabulous. When people truly feel for a popular artist - when they follow in their thousands behind Dickens' coffin or Valentino's - it is only the dues returned for pleasure given, and it never feels like enough. I would be hard pushed to think of another artist in any medium who has given me as much pleasure as the divine Ms H.

In fact, the marvellous weight of the pleasure ennobles all cliches, and I hope to see the obituaries full of "the last of her kind", "the greatest star in the firmament" and the rest of that sort of guff because, for once, it is all true.

© Zadie Smith 2003