‘I was really very taken with Al from the very beginning. Our relationship was absolutely wonderful in some ways, but is he the love of my life?

'No, not really, because he wasn’t the love “of my life”… He was the love of that time of my life. Each man had a different decade.

Woody was my twenties, Warren was my thirties and Al was borderline: late thirties/early forties.’ She adds a little wistfully: ‘I never see Al now.’

She still sees Beatty occasionally. ‘He was honoured recently [Beatty received the AFI Life Achievement award in 2008, at which Keaton made a speech] and I spoke about him. But he’s got a big family life and I don’t really run with his crowd.’

Of all the men in her life, it is Allen who seems to have had the most lasting impact. He becomes a leitmotif of our conversation – she refers to him at least six times, always with warmth.

‘I still love him – there are some people who stay in your life and it matters and they are in for the long haul. We talk often on the phone.’

Perhaps surprisingly, Keaton, now 67, has never married. ‘No one ever asked me,’ she says matter-of-factly. ‘I think they got to know me and realised, “Oh, jeez, she’s like…”’ She tails off.

‘I remember Woody saying, “Living with you is like walking on eggshells.”’

Why did he say that? ‘I think because I had too much sensitivity. I was hurt by everything… [She mimics herself] “They didn’t like me,” “Why am I not good enough?” “I didn’t get the part.”’

I guess that’s why you were so perfect for his films. ‘Yes,’ says Keaton, with a laugh: ‘Neurotic.’

Keaton is fun to meet. One gets the impression that, unusually for an interviewee, she actually enjoys talking. T

he only problem is trying to keep her on track as she lunges down conversational side alleys and stream-of-consciousness musings – like this random piece of information when we are discussing marriage:

‘When I was 14 this girl called Leona Kramer told me that intercourse was like going to the bathroom backwards. That has always stayed in my mind. Why? She wasn’t even a friend. It’s so absurd the way the mind works.’

When she was younger, says Keaton, she longed for marriage. ‘I remember watching [the actress] Myrna Loy being interviewed and somebody said, “It’s too bad you’re not married anymore.” And she said, “I don’t want to be married anymore – not at all.”

'And I remember thinking, “How sad, how could anybody come to that?” I was in the prime of my life, you’re biologically driven in some way, you can’t help it, you’re an animal. So I looked at her and thought, “That will never happen to me.”

'But you know what? It has happened.’

Warming to her theme, she continues, ‘I’m an old maid.’ Triumphantly, she repeats it, ‘I am an OLD MAID.’ She roars with laughter.

‘That concept is such nonsense – the idea that if you never marry you’re destroyed. I can’t imagine marrying now, that’s not happening.

'I can imagine a companion, someone who says, “Good to see you, what’s going on?” But, then, if it’s a man, are you supposed to have a relationship? Share a bed?’ She shakes her head.

Although she came to terms with never marrying, she could never give up on having children. So at the age of 50 she adopted a baby girl, Dexter (now 17), and a few years later a boy, Duke (now 12).

She says that much as she adores them, the situation is far from ideal.

‘I have a lot of mixed feelings about it. I do think it’s important to have a male presence, a father who loves those children. And age-wise it’s better to start younger. I’m 67. I’d like to be younger right now for them.

'You have less time and you have to think about that. You want to be around for their twenties because that’s a tough time.’

Keaton is the first to describe herself as something of an oddball: ‘I’m a little bit of an exhibitionist. I do things that are a little bit off the beaten track.’ Even her dress sense displays that.

When she received the Oscar for Best Actress 35 years ago, critics said she looked like a ‘bag lady’.

Unlike the other immaculately turned-out actresses in flowing dresses, Keaton had thrown together an outfit of two skirts layered on top of a pair of trousers, a white shirt, a black string tie, a scarf and her sister Robin’s socks.

Although always pretty, Keaton was never a classic beauty. ‘I’m not beautiful and I never was. But I did really try my best to get as much attention as I could to make up for it.’

Today her choice of clothes is still eclectic. She is wearing a camel-coloured jacket and trousers from Céline (‘On sale,’ she points out). The rest of her outfit is ‘my junk’.

The jacket is cinched in with a broad black belt and underneath is a white turtleneck with sleeves that loop over her thumbs.

She is wearing black heels and black-and-white spotted socks that match her alternate black-on-white and white-on-black spotted nails.

‘Can you see how I jerry-rigged my nails?’ she says delightedly, splaying out her fingers. ‘I put Sally Hansen stickers on them. True, they don’t exactly match…’

Her fingers are weighed down with knuckleduster rings and in each ear she has three studs. ‘I love clothes,’ she says. ‘I dress myself,’ she adds, meaning that, unlike many Hollywood stars, she does not have a stylist.

If she hadn’t met Woody… Before the words are out of my mouth, Keaton interrupts: ‘Nothing. Zero,’ meaning she would never have made it without him.

As a girl growing up in California, the daughter of Jack Hall, a real-estate broker, and his wife, Dorothy, an amateur photographer, Keaton had idolised Allen for years before moving to New York to study acting at the Neighborhood Playhouse and to moonlight at nightclubs with a singing act.

She met Allen when she auditioned for his stage production of Play It Again, Sam in 1968. She fell for him instantly.

‘I wanted to be his girlfriend,’ she says bluntly today. ‘So I worked at it.’

The feeling was clearly mutual and for years they were an iconic couple – Allen has even credited Keaton as his muse in his early film career.

She played eccentric characters in several Allen films including Sleeper,Love and Death, Interiors, Manhattan and, of course, Annie Hall.

The character of Annie Hall – complete with kooky mannerisms and self-deprecating sense of humour – was based on Keaton herself.

Winning the Oscar for Annie Hall was overwhelming: ‘I was like a deer in headlights – like Jennifer Lawrence when she fell down [as she went up the steps to receive her Oscar]. It was too much. I didn’t know what to make of it.’

She describes Woody as ‘a huge mentor’ in her life. ‘He really taught me the work ethic, I applied his principles because I saw how he lived.

'He really is the most disciplined person, very work-oriented. I remember there was a period when I was with him and he read all of Dostoevsky and then all of Tolstoy, he was learning French and playing the clarinet every day.

'And look at him now – he’s still making a movie a year.’

Keaton is prolific, too, having acted in more than 50 films while also producing, directing and screenwriting. She has been a blogger at The Huffington Post and a face of L’Oréal since 2006.

In addition she is a photographer, singer, author and property developer, renovating and redesigning fabulous houses.

Madonna was one of her clients, buying a Beverly Hills home from Keaton in 2003.

Of her fascination with visual arts and design, she says, ‘I love pretending what a lifestyle is. You just let your mind go and picture a dream. I’m very interested in beauty, I’m writing a book about beauty at the moment.’

Life becomes easier as she gets older. ‘I’m more impulsive. I just feel, “What’s the difference? I’m going to say what I think.” I'm more outspoken, less afraid.

'It used to be that every movie bore such weight. Now I think, “Who cares?” You’re going to be criticised whatever you do.’

In fact, she is more often praised than criticised. She describes the 2003 film Something’s Gotta Give – in which Jack Nicholson’s character falls in love with her – as ‘a miracle’.

‘Nobody was hiring me to play the romantic lead in a romantic comedy. I was 55. That was a once-in-a-lifetime situation.’

In her latest film, her sense of comic timing is as acute as ever. Her co-star is Robert De Niro.

‘I made a movie [The Godfather: Part II] with Robert De Niro about 20 years before and I just remember being terrified of him, I was so in awe of him.

'I had a massive crush on him and I thought, “Oh God, maybe he’ll ask me out.” You know, he didn’t even look at me, I was not his type.

'Then I did this other movie called Marvin’s Room, and he was the producer and he seemed unapproachable to me because he was such a legend.

'Then with The Big Wedding I had to do more with him [not least quite a noisy sex scene]. I thought, “He’s changed.” He just seems much more empathetic and more like everybody else. He doesn’t flaunt his Robert De Niro-ism around. I really liked him.’

As for herself, Keaton says that she is a performer in every aspect of her life. ‘I hog conversations, I laugh too hard…’

I ask if she looks back on her life so far as happy. ‘I don’t think of it like that. More and more I think of life – as I look at that ocean out there – as a complete mystery.

'I feel in awe of the whole thing. There is no explanation for anything at all, nothing to take for granted. Zero.’

To demonstrate her point, she says she has been worrying all morning about a news report of a plane that is in difficulties. She doesn’t know anyone on board but her powers of empathy make up for that.

‘Its landing gear isn’t working and I’m sitting here and it might be crash-landing now. Those poor people inside. The wonder of it. The mystery of it.’

As she frets, her tattooed make-up artist comes running over to our table, saying he’s just heard that the plane has landed safely. ‘Oh, thank God,’ she says, visibly relieved.

Despite the ditzy image, Keaton clearly has a sliver of steel. When I ask if she ascribes some of her career to luck, she says, ‘Hell, no. I really wanted it. I went for that dream.

'I had a lot of failures, I didn’t get what I wanted and I kept going back for more. If I couldn’t get the lead, then I’d try the chorus and if I couldn’t get into the chorus, I’d try something else.’

She says she is no longer seeking the impossible dream of the perfect life. ‘Going after dreams can be very dangerous. Dreams can turn on you. From now on, I want to look for reality.’