Diane Keaton, the Oscar-winning star of Annie Hall, Father of the Bride, Something’s Gotta Give and other classic films, died last week on 11 October in California, at the age of 79.

Keaton wasn’t merely famous, she was beloved. She took things that usually make people uneasy – nerviness, odd clothes and emotional honesty – and made them winning. She turned eccentricity into good manners. Where most actors work to seem spontaneous, Keaton seemed to have been born that way.

Keaton was the rare kind of actress whose intelligence never alienated men and whose beauty never turned away women. To watch her was to be reassured that life itself could still be fair. She came of age in an era when psychoanalysis became chic and talking about your feelings over lunch was the done thing, and turned these modes of introspection into a social grace. With her characters, worry was a personality trait rather than a diagnosis. They didn’t hide their anxieties, they arranged them tastefully.

As a student, Keaton studied the Meisner technique, developed by the hugely influential acting teacher Sanford Meisner. (He also recommended her to Woody Allen for his production of Play It Again, Sam.) Meisner’s methods value instinct over polish and foster reacting over pretending. Keaton turned this discipline into lightness. Her spontaneity was not random but highly finessed. She used uncertainty deliberately, as a language. This is why her characters never seemed written – they simply happened right in front of you.

What made her particular persona work so well on screen was that it was not a set of affectations superimposed on top of technique. Rather, her persona came out through her acting. Her timing was like jazz, with lines delivered half a beat off. Her gestures seemed accidental but landed like choreography. In Annie Hall, the pauses were punctuation, the flustered little noises, subtext. She looked as if she were discovering her emotions at the same moment we were, which is as close to cinematic truth as anyone can get.

Annie Hall fixed her image as the intelligent, chaotic woman whose sincerity makes her irresistible. The persona was so distinct that it became hard to tell from the outside where the character ended and Keaton began. Woody Allen, writer and co-star of Annie Hall, denied any strong autobiographical links between their story and the characters’ – but the myth stuck because we needed it to. Rather than fight this process of identification, she refined it. Each role after Annie Hall was a variation on the same melody: a woman balancing wit and vulnerability, without ever losing her composure.

Keaton didn’t just play an archetype, she became one, too. Every smart, self-deprecating heroine since – from Nora Ephron’s to Greta Gerwig’s – owes her something. The hats, the laughter and the trembling candour all still circulate through film and fashion to the point where people don’t even realise who they are imitating.

In The Godfather films, Keaton was Kay, the quiet conscience amid the corruption, a study in stillness that made the other performances seem theatrically overblown. In Reds, she gave idealism a human face. In Baby Boom, she turned working motherhood into an aesthetic. Father of the Bride softened her into the nation’s favourite worrier. By Something’s Gotta Give, she had evolved into the ultimate adult: luminous, self-aware, and utterly at ease with her own contradictions.

The through-line to all her performances was authenticity – performed authenticity. This was the paradox she mastered. The stammers, the pauses, the flutters of self-consciousness, all served a discipline so precise you could miss it. She made acting as effortless as personality, and personality as interesting as art.

Keaton aged as she acted, gracefully and with wit. Her death feels oddly personal because she stood for that rare commodity: the possibility of adult contentment. She showed generations how to be themselves without apology and without earnestness. While she has gone, she leaves behind the archetype she perfected. And now the world feels a touch less forgiving – and much worse dressed.