Over the course of an hour I get the full Diane - the exquisite timing, the dramatic pause, the droll shrug, the sotto voce, the under-the-eyelashes withering glance. The sense that actually her mind is somewhere else completely, probably buried deep in a photography book in a store in Greenwich Village.

Plus, she's wearing a hat - a cream fedora - which is what you want from Diane Keaton. Also a striped C??line shirt with a wing collar and black high-waisted trousers that swoosh over high heels. The look is finished with more than half a dozen large silver crucifixes that she's picked up at "the swap-meet" (flea market) over the years. Like I said, the full Diane.

As she pulls answers out of the air, switches from high energy to reverie, clinks her bracelets and fidgets with her black onyx rings, you just want to sit right there and watch. Which is presumably why she's had a near halfcentury Hollywood career playing the woman it's impossible not to fall for.

She's played at being in love at every age from 20-something onwards on the big screen. "Yeah, think about that!" she snaps. "With all kinds of men. It's been fabulous! It's the perfect relationship. I don't have to have the relationship but we tell these stories and then they leave. Perfect! I've got to play around with all different kinds of men."

This time she plays around with Brendan Gleeson in Hampstead, an unlikely romance set in the ponciest of London enclaves. It's a love story that traverses the ground between Notting Hill and Something's Gotta Give, with Diane's widow Emily feeling down in the dumps and past her prime when she encounters Brendan's Donald Horner, an Irish recluse who has been squatting for 17 years in a Hampstead Heath shack and is about to be evicted. In him, Emily sees the opportunity to give her heart to both a cause and another person.

"Well, I mean, it just gives hope!" she exclaims about her reasons for taking on the movie. "To people who are alone, or they've fallen apart, or they've got older??? Or your husband dies and you find out that he cheated on you and you owe money. And you're lying to everyone and you're not really friends with Lesley Manville???"

Manville plays Fiona, Emily's neighbour and No. 1 frenemy, in a cast that includes a host of UK national treasure types, from Jason Watkins, last seen attempting to saw up Thandie Newton in the British cop show Line of Duty, to James Norton. ("He's the next James Bond, right? Well, I mean he should be," says Keaton wryly.)

The film is based on the true story of Harry Hallowes, a man who did indeed set up home on the heath, although the real shack was not quite so Pinterest-friendly as the tumbling-rose, shabby-chic idyll, filled with books and candles in wine bottles, the film conjures up. But how else to lure the cultured, enviably clothed Miss Emily, as Donald calls her in a strange feudal throwback? Emily and Donald embody the old-fashioned essence of romantic comedy - opposites. It's a concept that Diane has always understood.

"Oh yeah, I think your opposite stirs you into something. They shake you up and that always is attractive." Pause.

"For a while." She laughs. "But you want to be companions. You want to enjoy yourself with that person without having conflict all the time. Conflict can be sexy but it's exhausting."

So what's her idea of a good real-life relationship? "Well," she says, dropping her voice and looking directly at me through her spectacles. "You're talkin' to the wrong person."

Though she's played at being in love and been in love with some of the most famous men in the world - Woody Allen, Warren Beatty, Al Pacino - Diane has never committed to anyone for long enough to permanently change her relationship status.

"You know, my father was a 'greeter'," she says, by way of explanation. She puts out her hand and pulls on a wide Jack Nicholson grin. "'Jack Hall!' he'd say. Charming, great smile. But he could only sustain it for a short time. And I remember Warren [Beatty] said to me, when I was with him, 'You know, you'd make a great politician. You can do it for a coupla hours and then you gotta get out.' And he's dead right. I couldn't sustain things that were more personal."

The actor refers to her late parents a lot, particularly in relation to the characteristics she inherited from them. "They were very charming but they weren't very social people. And that used to worry me when I was younger. 'Gee, why aren't there more people around?' And now I'm totally them. Because I don't want it for very long. Warren nailed it!"

Nevertheless, at the age of 50, Diane realised that she did want another kind of connection. So she adopted her daughter Dexter, now 21, and later her son Duke, now 16. "I figured I had to do something because, you know, what was I doing aged 50? I was just being." She was lonely?

"I was worried about what my engagement would be with people in the future. Was I going to be engaged enough? And, of course, the kids forced that engagement. With kids you're back there in the middle of it."

She says she "identified immediately" with the character of Emily and her confusion about life, and she understood Donald's situation. "Both of them are so stuck. He can't just stay in that shack for another 15 years - what is he thinking? And she is passive and stuck and doesn't know what to do.

"Many people do feel stuck in their lives and they don't know how to make a change. And not only that, they don't want to address it because it's too scary." It's hard to believe, I say, that a woman with such a diverse career and life as Diane - a woman who's not only appeared in over 45 films but written seven books (her latest, The House that Pinterest Built, out in October, is about her new home), been a photographer, had those high-profile relationships, started a wine label and brought up two kids by herself - could possibly identify with "stuck". She seems always to be embarking on a new adventure. She looks away and looks down with a smile. "Yeah, but I'm not really. Not really."

Diane, born Diane Hall, was brought up by Jack and Dorothy in Highland Park, Los Angeles, along with two sisters and a brother. Her father studied to be an engineer, and when Diane was eight her mother won Mrs Highland Park, an all-American, all-1950s-style competition for the best local wife.

Seeing her mother - "she was great-looking" - on the stage at the local theatre surrounded by "a cornucopia of gifts" was for Diane the catalyst for a career in the spotlight. "I wanted those gifts, I wanted to be up on that stage," she says. "Although, turns out I didn't want to be on stage but I did want to act."

Success, she says, came because she was lucky enough to find Sandford Meisner, an acting teacher whose technique suited her, and who encouraged that loose, naturalistic style that is so identifiably Diane.

"It was about just being in the moment with your partner," she says of Meisner's method. "It's all about the person you're acting with and paying attention to them. It's not just about telling the story but experiencing it through your responses to them. But it can get a little sloppy, a little stuttery."

???You don't really see that style from Diane in her first major film, The Godfather, in 1972. She says she never understood why Francis Ford Coppola cast her in it until she watched it again at the recent anniversary celebrations in New York, with the original cast.

"There were 6000 people there and seven of us up on stage - De Niro, Pacino, Jimmy Caan, Robert Duvall, Talia Shire, Francis and myself." She thought she was a weird choice because she was known "and even Francis had said it, as an eccentric".

She pauses. "Well, yes, I was. But watching those first few scenes with Al and I shopping, or at the wedding, I see that we're on the outskirts and we're kind of the people that aren't 'in'. So it was good casting for me because I had no voice at that time in my life, in that movie. I was overwhelmed by this big world and didn't know what to make of it. And I look so weird!"

For all its success, she says the film didn't help her career. But when Woody Allen cast her in Annie Hall, for which she won an Oscar, everything altered. "Oh yeah, Annie Hall changed my life. Because when Woody said, 'Just dress like you do,' it was the gift of all time. It was the greatest gift. Now he makes fun of me. He thinks I'm a freak. But that's our relationship. I tell him what I think of him, too."

With her mannish trousers, waistcoats and floppy hats, plus that scatty, chatty style of acting and Allen's script, Diane created one of the most magical screen characters in movie history. Her personal style was key to how we thought of the character, but she brushes off the idea that her style was somehow special.

"It was just developing all the time, from the street but also fashion. I mean, in the 1970s, Comme des Gar??ons - I would go and stare in the windows. Romeo Gigli, he had a flair for tailoring. I liked Ralph Lauren early on, too. Because that look gives me a body, that's the way I feel about it. Because I'm kind of straight up and down, I love it."

She made covered-up look sexy and still does, I say, thinking about the roll-neck tops that have become her trademark - and which Jack Nicholson famously scissored off in Something's Gotta Give. "I've always been quite covered up," she concedes. "Oh yes, I believe in that!

"The other person I think is the biggest genius of all for covering up is Karl Lagerfeld. I feel like I should meet him. I'm sure he wouldn't like my style but I have to say when I look at him, oh wow, oh honey. He took it far and I think yeah, why not? He looks good, man! Spec-tacular. So I like to think of myself as the female Karl Lagerfeld." And she has a good laugh at that.

Hampstead is released on August 17.