If Keaton’s got a few spare hours, we’d be glad to explain it to her. Since she la-di-dahed her way into the hearts of audiences (and the Academy) with “Annie Hall” in 1977, she’s charted one of the longest and most diverse careers of her generation, playing comedy and drama with equal grace: “Looking for Mr. Goodbar,” “Reds,” “The First Wives Club,” “Marvin’s Room,” “The Godfather” trilogy. She’s been hilarious and heart-rending–often at the same time–and perhaps never more so than in “Something’s Gotta Give.” Despite the comic setup, the movie, opening Dec. 12, is really about her character–a long-divorced playwright who finds herself, against her wishes, emotionally vulnerable again.

“I loved playing being in love,” Keaton says. “That was a beautiful experience because it’s so out of keeping with my life.” Although Keaton has had major romances–Woody Allen, Warren Beatty, Al Pacino–she has never married and, at 57, doesn’t expect to. “Romantic love,” she says, “I was never comfortable with it, ever. It was always fraught with anxiety. I always wanted too much for myself, and it brought out too much fantasy for me about the way it should be. So acting gives me the opportunity to pretend that I am somebody who’s open to the possibility.” And acting with Keanu Reeves, who also pursues her in the new film, didn’t get her thinking about younger guys? “I like to look at them,” she says. “What’s not to enjoy? But I don’t want to go there. It’s too weird. When I made ‘Mrs. Soffel,’ I got to do these screen tests with all the guys–Kevin Costner, Tom Cruise–and kiss them. The worst was kissing Tom Cruise, because his body was like a rock, and I thought, this is just wrong. This doesn’t make me feel happy.”

Not that she’s got time to date anyway. She’s mother to an adopted daughter, Dexter, 8, and a son, Duke, 2. She publishes books of photographs, both her own and other people’s; she’s fascinated by design, finding and remodeling a series of homes; she’s been a fashion icon for decades, wielding her trademark style of swap-meet sophistication. “The thing I was not prepared for about getting older is that I feel more engaged in life than ever,” she says. “More things are pivotal. I lost my father. My mother is not totally well. I’m raising children. It’s overwhelming, in a sense. How can you address what you have in front of you and stay vital?”

Nicholson figures she’s vital enough for both of them. “She’s a fascinating woman,” he says. They’d shared scenes in the 1981 drama “Reds,” but they hadn’t spent time together until this film. “She has tremendous energy. A lot of opinions. It’s always good to be around someone who’s questioning what’s going on in life. And she’s a wonderful actress–authentic and unpredictable. Her presence is very specific and very powerful.” She’s looking pretty great, too. “Yeah,” he says. “Keaton knows how to pull it together.”

In the film’s most heartbreaking scene, Keaton tells Nicholson that she loves him, knowing that he doesn’t love her. “It was stunning,” Nicholson says. “I completely believed that she loved me, certainly for the moment.” And she did, in a way. “He’s dazzling,” Keaton says. “That face, it’s irresistible to me. I have never spent that much time in bed with anybody on any movie, and we would talk about former love affairs and alliances with people. We really got to know each other, because it’s so humiliating. You’ve got this apparatus to hide your breasts and you’re thinking about your stupid little arms.” She laughs. “That’s the joy of regular life: you don’t have to think about your body in the morning.”

Keaton had still more reason to think about that: in “Something’s Gotta Give” she has a brief full-frontal nude shot. But she didn’t hesitate about exposing her body to moviegoers. “At this point, does it really matter?” she says. “It’s not a precious item. I used to be so fearful–like, ‘Oh, God, it’s special’–but nobody is looking at me the way I once imagined people would look at me, like with deviant thoughts. I think they just go, ‘Huh. There it is. Intact’.” Hardly. The reaction is more like, Wow. Who knew? “Well,” Keaton says. “For a second it’s fine.”

And so is she, by the way. “I have huge, big, important sustaining loves that make my life valuable, necessary,” she says. “But happiness? I don’t know. Isn’t life really a series of problem-solving events? It’s how much you’re engaged by the fun of solving your problems, and continuing to be curious. That’s as much as you can expect.” Keaton smiles. “I feel like I’ve done a hell of a lot with what I am. I’m quite an ordinary person, but I’m extraordinary in the sense that I try very hard. Whatever opportunities came my way, I held on for dear life.” Now she laughs. “I really wanted a lot.” And she’s gotten almost as much as she deserves.