Sharon Stone is crying softly. There are, I am learning, several different Sharon Stones and, right now, I am being treated to the emotional, caring one, as she discusses her role in Bobby, an all-star ensemble piece that re-imagines the night that the presidential hopeful Robert F Kennedy was assassinated, in June 1968. Later, I will experience the cold, brusque Sharon when she expounds her opinions on world peace, and sundry other matters. Of course, we're all familiar with the man-eating Basic Instinct Sharon, as well as the dazzling red-carpet edition - that one's a life-sized Barbie doll that comes with a selection of outfits; the actress comes with a collection of very definite moods, instead.
It's a bravura performance that she's giving at the Beverly Hills Four Seasons Hotel. "We cry every time we talk about this movie," she explains through sobs, although its not clear who the "we" is. Stone seems to be referring to the Bobby director-writer Emilio Estevez, although Lindsay Lohan is snuffling in tandem at the end of the table. Later on, in the midst of her co-stars - Lohan, Elijah Wood, Laurence Fishburne, Demi Moore, Emilio Estevez and Helen Hunt - earnestly discussing Bobby Kennedy, Stone will, bizarrely, launch into a lengthy discourse about coats and shoes.
"[There are] people who are homeless. People who cannot afford a coat and so they're dropping out of school. Kids who are not going back to finish their senior year, kids who are not going back to high school because they do not have a coat to go to school... I've been working with a man at the Burlington coat factory and we've been doing these projects all over the Atlantic north-east because there's so much of this situation where children are leaving high school because they do not have a coat... So when you vote, you need to really think about what you see on TV and what's really happening. Because my family is out there putting shoes and coats on these kids who are not going to school because they really don't have them.
"That's what's really happening in America today," she concludes. You can almost feel the mute panic emanating from the assembled elite of Hollywood publicists.
Together with Anthony Hopkins, Harry Belafonte and Demi Moore, Stone was one of the first actors to sign up for Emilio Estevez's low-budget film, which follows the stories of 22 fictional characters at Los Angeles's Ambassador Hotel on the fateful night. The actress accepted a union-rate salary for her services. Estevez encountered a very un-diva-like Stone. "She said to me before we began filming, 'I don't care if I'm fat or thin, glamorous or unglamorous - I just love this role'," he recalls. "I think people will be surprised by her performance, where she's gone a little deeper than we've seen her go before."
Stone remembers that, when Estevez called about the movie, "I felt so lucky that he offered me the part of this lady who ran the beauty shop, and who got to interact with all these different characters and had such humanity". Her performance as Miriam, a hotel hairdresser and long-suffering wife standing by an unfaithful husband (played by William H Macy), is one of the best in the movie. Miriam's salon is a haven where Moore's and Lohan's characters feel free to pour out their problems. Has Stone ever bared her own soul under the ministrations of her hair-stylist? "Good Lord, no. I know better than that," she scoffs. "I would say I'm a person that people tell their troubles to, while at the same time, I've certainly had troubles of my own. Louis Armstrong sang it so well, 'Nobody knows the trouble I've seen'. I think we've come to a time when people have forgotten that character comes from standing through difficult times, and that, when you agree to stand by somebody, a relationship grows, and becomes richer and deeper when you grow through people's failures, not just their successes."
Her remarks, uttered with passion, appear incongruous in light of the demise of her five-year marriage to the San Francisco newspaper editor Phil Bronstein in 2003, in the aftermath of Bronstein's 1999 heart angioplasty, Stone's 2001 brain aneurysm, and the bizarre incident in the same year when Bronstein's toe was crushed by a Komodo dragon at Los Angeles Zoo. "So often now, people just turn their back and walk away when someone fails or changes," the thrice-wed actress (who has at least three adopted sons, including Roan, six, Laird, 20 months, and Quinn, nine months) continues, blithely. "And really good relationships, relationships that are rich and textured and alive, are the relationships that are built on acceptance and learning, and you accept and learn when people fail. Those are the relationships that have quality and dignity and growth, and I think if we can learn anything, it would be that. And when you're with a person for a very long time, I think you develop another language that isn't everything that's said with words." Stone was 10 years old at the time of Senator Kennedy's assassination, growing up in Pennsylvania. "I remember all of it," she sighs. "I remember when JFK was killed and Martin Luther King. It was so awful. I was asleep when Bobby was shot because it was the middle of the night for us on the East Coast. I don't remember how I felt. I think the Sixties were a time of turmoil and difficulty. For me, I was a very political kid, and I really remember the Vietnam War. You know, I was an artist, and a painter and a pottery maker, and a sculptor. I sold hot dogs at baseball games and we passed out political buttons. I was a hippie kid."
Today, as a woman who turns 50 next year, it must be said that she looks very good. "To look good starts from the inside, so you need to be honest. Lying can make you look bad. I don't use caffeine, I very rarely have alcohol. I believe in eating in moderation," she told People magazine last month.
In many respects, Basic Instinct in 1992 can be regarded as the centre of her career, the point from which 14 years of bad movies span in both directions. Prior to Basic Instinct she showed up in such stinkers as Police Academy 4, and continued to make lousy movies afterwards. If her stellar performance in Martin Scorsese's 1996 Casino earned a Best Actress Academy Award nomination, then it proved a mere blip along the way. Last year's Basic Instinct 2 perpetuate her 12-year string of bombs, a $70m (£36m) film that earned $5m at the US box office. It's possible that Bobby will break her losing streak.
"The nice part of this period in my life is that I'm getting more interesting roles all the time," she says. "There's lots of interesting parts right now."
'Bobby' opens on 26 January