Diane Keaton: “If you’re happy, you’re mentally ill”

Then, it was all about pulling off that girl-dressed-as-boy schtick, which on her was impossibly feminine and cool… the wide kipper tie and even wider bell-bottoms, the little waistcoat over crisp white shirt and the men’s fedora.

The men’s hats remain her trademark – today it’s a pale one, with black trim around the tip and crown. The inspiration came from observing French actress Aurore Clément. “I remember her on the set of The Godfather [Keaton portrayed Kay, who married Michael Corleone, played by another ex, Al Pacino] and she was wearing one of the men’s hats, and I said to myself, ‘Oh, I’ve gotta get a hat like that!’ So then I would buy them from the Goodwill [an American charity shop] or at the swap meet [flea market].” Now Keaton has them specially made at Baron Hats in Burbank.

Diane Keaton with Woody Allen in Annie Hall

Her trousers are also quite Annie Hall – very tight around her pencil frame, fanning out around her high-heeled black sandals – cinched in with a belt as wide as a weightlifter’s, which a costume designer created for her. The black-and-white striped shirt with oversize cuffs and a stiff collar is one she has worn for a long time. What Keaton then does – which makes her so inimitable – is mess the whole thing up with huge knuckledusters on two or three fingers of fingernails, and clunking great crucifixes (“Cheap chains – and I found the crosses and just put them together”) swinging around her neck. It’s as though Axl Rose has been dressed by Chanel.

Her hair is shoulder-length and silver-grey, her eyes are well made-up behind black-rimmed spectacles, and she wears an expression that flits between amusement and bemusement. Part of her personality is to go big on self-deprecation and doubt, and that makes it hard for her to be good at publicity gush.

She’s a bit low-key when she talks about her new film, Hampstead (in cinemas from Friday 23 June), in which she plays a widow who connects with a recluse (Brendan Gleeson) who has built a house and lived self-sufficiently on the heath of the title for 17 years. “I liked the script by Robert Festinger because it was about a woman who is my age [71] and a lost soul, and then the miracle happens. Like it sometimes does to us. We do something and out of nowhere that becomes an opportunity to change. I love that.”

With Brendan Gleeson in Hampstead

I get the impression there is no romantic miracle in her life right now (she has never married) – although, perhaps, there is another kind of happiness in becoming a parent. Twenty years ago she adopted a daughter, Dexter, who is now 21, and later a son, Duke, who is 16. With all the ups and downs of motherhood, does she feel fulfilled? “Yes, but you also get moments of such concern and care,” she frowns. “It’s the strangest feeling. It’s making me sad thinking about it. Are they going to be all right?” She repeats, “Are they going to be all right?”

Keaton is either monosyllabic in her responses (Do you feel old? “Yes”) or discursive, and on awkward subjects is liable to turn into Annie Hall, with verbal advances and retreats and merry-go-round thinking. We talk about the bulimia she suffered from in her 20s when she was going out with Allen. “I was trying to fill a vast hole. I had a massive appetite for... everything. Just wanting a lot. I can’t explain it – I still don’t really know why. But I will tell you why I stopped. Because I went into analysis with a woman five days a week because it was real bad.”

Did Allen know about it? “No, not at all. No one knew. I was really good at hiding. But I asked him about an analyst. Yeah, maybe he did know, but I don’t know for sure. I think it came up in a conversation like, ‘Maybe I should?’ and ‘Do you know someone?’… that kind of thing...

“I didn’t tell my analyst for a year. One day I was lying on my back, not looking at her, and finally I’d had it with myself and I just blurted it out and I said, ‘You know what? I’m never going to change... I’m never going to stop...’ She didn’t say anything. She just let it pass and then I was able to stop because finally I confessed. I was a genius at hiding.”

1976: With director Milos Forman after he won an Oscar for directing One Flew Over the Cuckoo's Nest

Her diet continues to be unusual. “I quit eating meat and fish so it’s a lot about nuts and cheeses. I do love cheese. It’s weird, once you have stopped being bulimic – and it was a habit for three years – you’re strange about food.”

Keaton grew up in Santa Ana, California, as the oldest of four siblings. Her father Jack Hall was a civil engineer, her mother Dorothy a keen photographer. At 13 years old, Keaton begged her mother to pay for her to take acting lessons with Kenny Aiken, who put on local productions. “But Kenny had no interest in me and he would never cast me in anything, and I was terribly disappointed,” she says. “So then Mom spoke to Kenny and he told her that I needed to go to modelling school because I didn’t look good. That I should become more refined and feminine and more groomed. This drove me crazy, so I didn’t do classes any more.”

She talks about how her upbringing failed to prepare her for being able to socialise with ease. Therapy helped her to get over these antisocial tendencies but it was an effort. “I was never really quite ‘in’ – I think because it wasn’t a family trait. I like to be a little detached. I’m not a joiner-in. I did cultivate friends and like to do things with them but I still do spend a lot of time by myself.”

On the Jimmy Kimmel Show

A childhood memory resurfaces. The family would drive to Laguna Beach every weekend, as her father was a diver who loved the ocean. One day, they were on the beach and there was a party in a tent. “The people were drinking and laughing and I remember thinking, ‘Why aren’t we like that?’ And that was the beginning of my understanding that we were not really social. We were charming but not social.”

In her 30s, while living in New York, Keaton found one way of being social that wasn’t too demanding: volunteering at a Jewish home and hospital for the aged. “I felt I had to do something, and these kinds of situations are easy for me because it’s a limited time span and you can be nice and charming and friendly and be interested and then – go.”

Now she visits her brother, who is in the memory care area of an assisted living space in Culver City, near her home in the Palisades. “Not only am I seeing my brother but I’m meeting a lot of other people and that makes me feel of value.”

Her mother, she says, was “extremely artistic and lovely and encouraging all the time”. It was Dorothy who was a co-conspirator in helping her eldest daughter create her idiosyncratic style. “I remember being teased at high school for a dress that Mom had made me out of black-and-white polka-dot material with a big skirt.”

She has lived in London at various times and believes it to be the mecca for street fashion. “People don’t necessarily have a lot of money but they have brilliant imaginations and a lot of style.” In LA, by contrast, “none of it matters unless you’re Kim Kardashian. Then you’re going to get a lot of attention.”

Is she still insecure about the way she looks? “You always are,” she says. “I don’t think it ever goes away. I don’t think anyone is not insecure, do you? I think everyone is a bit... What would be really great would be to just not... But, yeah, of course...” Sigh.

We end with me asking if she considers herself to be happy now. She goes into a spectacular Annie Hall arc. “That’s just impossible – I don’t even know what that means when you ask someone if they’re happy – of course not. You’re not happy but you are engaged and there are things that are just miraculous, you know... A lot goes on in one day in our lives… You can be this and that… So I don’t know what to say about that.”

She stops and starts again. “It’s a ridiculous question because no one can really be happy – if you’re happy, you’re mentally ill. I mean, there’s a lot of sad things going on.”

Hampstead is in cinemas from Friday 23 June