SITTING cross-legged on her hotel suite sofa, Diane Keaton is exactly what you might expect from the actress for whom they invented the word "ditzy". With a long streak of silver hair, she’s wearing thick biker boots (clean, but currently resting on the soft furnishings), ripped jeans and an oversized belt clenching in her tailored jacket. Typically eccentric, “I’ll do whatever!” she says, fidgeting with all the restless enthusiasm of an excitable child.

Famed for her roles in Annie Hall and The Godfather, even the sight of coffee being delivered practically sets her into spasms. “Look at this…oh my god!” she sings, as the publicist brings in a tray, which he almost spills over my recording device. “That would’ve been it, it would’ve been over!” she cackles. “We would have to do this again!” You suspect she would too; Keaton loves to share – to friends, her therapist (“I’m way into it!”), to her followers on social media.

Now 71, she’s registered on just about all the major digital platforms – Twitter (she has 422,000 followers), Pinterest and Instagram. “I like Instagram the best right now. But I like Pinterest too – so I’m a little torn.” W magazine proclaimed her “the most underrated celebrity” on Instagram due to her oddball posts – quirky pictures of pets, shoes, wine (she has her own vintage, ‘The Keaton’) and art. She’s like that cool aunt you always wanted.

Keaton has always been obsessed by photography. Back in 1980, she published Reservations, a book of her documenting hotel interiors. She’s also hugely into design and architecture. She has bought, refurbished and sold numerous houses over the years – around twelve, she estimates. “That’s the problem! I’m a serial seller!” she chuckles. “I’ve lost a lot of money in real estate too. Oh yeah. I’ve restored houses at the wrong time and taken big losses. But I didn’t care.”

When she was growing up in California, her father Jack, a civil engineer, regularly invested in properties. “He loved real estate,” she says. Over the past two-and-a-half years, she built a house from scratch (with a little help, of course). “I went for brick. And it’s old brick – it’s from Chicago. I bought all this old brick out of Chicago and it’s got so much texture and it’s so wonderful, I love it. So I hope to do this again.”

Later this year, the resulting book The House That Pinterest Built will be published. But first she has a new film – oh, yes, she still finds time to act – called Hampstead. A true story, or at least part of it is, it’s based on the tale of the late Harry Hallowes, who began living on Hampstead Heath after he was evicted. Eventually claiming squatter’s rights, he won a court case and was awarded the land – said to be worth over £2 million.

Played by Brendan Gleeson, Donald (as he’s called in the film) becomes the focus for Keaton’s character, Emily, an American widow and local resident who spies him one day bathing in the lake. Out of this casual voyeurism, they strike up a relationship. “It’s like, ‘Oh my god, things like that can still happen in your seventies!’ Do you see what I mean? Don’t just think it’s over at any point in your life. You just don’t know what’s around the corner.”

The comedy is gentle, but it introduced her to the work of Irish star Brendan Gleeson, who she didn’t know. “I saw a lot of him in this movie!” she giggles, referring to his lakeside scene. “A big gorgeous guy…I love him.” Male friends are something Keaton has a lot of. “I won’t get to be friends with Brendan because he’s there in Ireland all the time. He’s not around. And he has a big family too. You need a friend you can spend some time with.”

Keaton, who never married, famously dated three of Hollywood’s leading men: Woody Allen, Al Pacino – her co-star in The Godfather saga – and Warren Beatty, who directed her in Reds, in which she played Louise Bryant, a journalist on the ground during the Russian Revolution. But rather than wed one of them, she veered away from a life partner, instead adopting two children, Dexter, now 21, and Duke, 16. Again, she’s hardly wilted from the experience.

She’s also good friends with Jack Nicholson, with whom she co-starred in Reds and Something’s Gotta Give (both of which gained her Oscar nominations) though she hasn’t seen him in a while. “He’s just kind of a genius,” she says. “You should be listening to him talk, because it’s the most entertaining experience of your life. If only I had a recorder! That’s the way I feel about it. If you could let me just record him speaking, I’d have a great book, I tell you.”

Keaton already has several of those, including autobiography Then Again and Let’s Just Say It Wasn’t Pretty, which deals with issues of ageing and beauty. In Then Again, she’s very complimentary to her mother Dorothy (whose maiden name Keaton she took for her own stage-name). Like Keaton, her mother was an amateur photographer. “Oh, yeah, I inherited [that], sure. My mother, she loved the arts and she was a collage artist.” Her brother Randy became one too.

“I would never have been a visual artist because I have no skills, no skills at all,” she says, modestly. Instead, Keaton began her acting journey playing Blanche DuBois in a school production of A Streetcar Named Desire. An early Broadway audition saw her rejected for being “too tall and too kooky” but it didn’t take long before she scored a role in the musical Hair. From there, she won the female lead in the stage version of Allen’s Play It Again, Sam. “Here’s to you!” she trills. “Thank you Woody!”

She and Allen dated for just a short time, but their working relationship spanned eight films, up to 1993’s Manhattan Murder Mystery. Before she made Allen’s 1977 classic Annie Hall, winning her a Best Actress Oscar, her films “didn’t get any attention”, she claims. Not even The Godfather or its sequel, in which she played Michael Corleone’s wife Kay? “But not me. Not me. Me in Annie Hall, I got a lot of attention. Then I won that award and that made all the difference and that gave me other movies.”

Even through this glorious period of her career, Keaton suffered from bulimia and low self-esteem. She doesn’t hide her neuroses. Even now, while she still acts – her next movie is Book Club with Jane Fonda – she finds it stressful. “There’s a responsibility [with acting]. You’ve got to take your hits,” she says. “I get really anxious about it…but then on the other hand, you’re telling this story that’s amazing in some regards. It’s distilled with everything.”

If anything, Keaton is the perfect example of how to defy ageing – just keep on working, creating, talking. Just not driving; that really freaks her out.

“What I’m terrified of, literally terrified of, is people on bikes and motorcycles...I’ve seen more accidents with people...on the road, on the freeway, on the f***ing motorcycle,” she rants. “I really worry about that when I’m in a car – it’s a weapon.” So maybe don’t get behind the wheel with her.

Hampstead opens in cinemas on June 23.