But now she is a British citizen. "I was sworn in the day Prince William and Kate were arriving in LA ? the day before my mother?s birthday, on 9 July. She would?ve been 71," says Moore, close to tears. "I?m going to get choked up now."
We are both silent for a moment as the actress composes herself. Does the grief get easier with time? "It?s still the same, it?s very painful," says Moore quietly. "I don?t know that you ever recover from loss. My mother was very young, too.
"She died at 68 [of an infection followed by an embolism]. The thing about having a very young mother who had you at 20 is that you expect that you?re going to be old ladies together," she trails off. "I?ve learnt that you have to appreciate every minute. Don?t wish your life away. Be present."
Moore looks lovely, but quite different from the sophisticated figure she cuts on the red carpet. She?s wearing no make-up, not even a dab of mascara, and is dressed casually in skinny jeans, a striped T-shirt and Tory Burch flats. Her long, red hair frames a pale, lightly freckled face and vivid grey-green eyes.
A waiter has saved her favourite table tucked away in a corner by the window. Moore turns off her mobile and orders mixed berries with fresh cream followed by an egg-white omelette.
Her philosophy extends to ageing. "If you?re 50 you?re never going to be 50 ever again so enjoy being 50," she says. "If you sit through the year wishing you were younger, before you know it it?s going to be over and you?re going to be 51."
In fact, Moore looks younger. She told me in a previous interview that she hadn?t had plastic surgery. "No, I haven?t had anything done. See, I have lines right here," she says now, pointing to faint creases around her eyes and mouth.
She puts down her fork, pausing to twist her hair back into a tight bun, then traces her chin with her fingers. "There are things about my face that I?m not crazy about, but you are who you are.
"It seems like everybody has done work,? she goes on. 'I see a lot of it, but I don?t think it looks any better. You?re not suddenly going to look 25. I?ve always been somebody who?s acutely aware of my mortality.
"You never know when it?s all going to be over. Being middle-aged,? she says emphatically, 'is about realising that you?ve lived most of your life. You don?t have as much time in front of you as you have behind you."
Moore was born in Fort Bragg, an army base in North Carolina. The family (Moore has a sister, Valerie, and a brother, Peter) moved more than 20 times during her childhood.
"We were all over the place, Nebraska, then Alaska, then New York, then Virginia, then to Germany when I was 16 in 1977. It was very hard, I don?t recommend it. I think moving around so much makes you insecure socially."
There was a positive side, too, she says. "It makes you adaptable. And you?re always searching for the universality of human behaviour ? which you do in acting, too, asking, 'What?s the thing that we all relate to? How are we the same?'
"Also, it was a great time to see the world because each country had a very individual identity. We?re living in a very global world now, but then you?d say, ?Look at these boots, I got them in Paris? ? and no one else had them."
Moore wasn?t a confident child. "I didn?t mind my hair but I just hated my freckles. Kids would say things like: 'Are you dirty? Do they go away?'" The experience inspired her to write her children?s book series, Freckleface Strawberry. The latest installment is out next month .
It was Moore?s love of reading that propelled her into acting. "I read everything Louisa May Alcott ever wrote. When I started acting after school it was an extension of reading. I am not a natural performer but I really like to pretend."
She studied drama at Boston University, then worked as a waitress to support her acting and spent most her twenties working in theatre and television. A stint on the soap opera As the World Turns led to a Daytime Emmy Award.
Robert Altman?s Short Cuts (1993) gave Moore her first film break. But global success came relatively late. There were three defining films that she says changed her status from supporting player to leading lady: Safe, from the director Todd Haynes; Nine Months, with Hugh Grant; and Louis Malle?s Vanya on 42nd Street (all in 1995). The thriller Assassins followed, then came Steven Spielberg?s dinosaur blockbuster The Lost World: Jurassic Park (1997). "Suddenly I had a commercial film career," she says.
"I don?t think that early success really serves anybody. What are you going to do for the rest of your life? What do you aspire to? It?s all about doing work because you like doing it. That?s what will sustain you, rather than doing it for fame or recognition."
She has enjoyed plenty of both, with seven Golden Globe nods and Oscar nominations for Boogie Nights (1997), The End of the Affair (1999), The Hours and Far from Heaven (both 2002). Although, surprisingly, not for The Kids Are All Right (2010), in which she played Jules, a lesbian mother of two, opposite Annette Bening.
In Crazy Stupid Love Moore plays a woman who is bored with her comfortable but dull 25-year marriage to Steve Carell and has an affair. "I loved the fact that on the first page of the script there are the words, 'I want a divorce.' You generally don?t see anything that bold in a studio comedy. It didn?t pull any punches."
The comedy works well, says Moore, because it is authentic. "I think marriage is really hard. My husband and I have been together for 15 years, which is hard for me to believe."
She shakes her head. "We talk about having to make an effort, go away together, do something different. But you have jobs and kids and the roof is leaking and then something breaks in the basement and then the dog has got some horrible virus and you suddenly think: this is what my life has become."
Crazy Stupid Love doesn?t sentimentalise either sex or relationships ? though the romantic notion of soul mates lies at the heart of the film. Moore is ambivalent on the subject.
"It?s rare to fall in love and so when it does happen it?s a pretty big event. How many of your single girl friends say, ?I can?t meet anybody?? On the other hand, marriage is about the accumulation of time, an investment in each other and in your children, not just the romantic part.
'There are people who hit the wall and split up and others who hit the wall and don't split up. One thing I?ve noticed is that you don?t avoid hitting the wall; you have to work at it."
But, she adds, "I am not saying that unhappy people shouldn?t split up. I was divorced. I got married too early and I really didn?t want to be there," says Moore of her marriage to the actor John Gould Rubin ? they separated in 1994. She met her current husband, Bart Freundlich, on the set of his film The Myth of Fingerprints in 1996. "You have to want to be married."
She reminds me of a moment in The End of the Affair. Her character, Sarah, is married to Stephen Rea and has an affair with Ralph Fiennes. "When Sarah dies, Ralph talks to Stephen about how hard it is for him. He says, 'She could shop and eat and cook with you but she could only make love with me.' They were in love, but the relationship was incomplete because they couldn?t share their lives. I was really struck by that."
Family life in Manhattan, while obviously privileged, is "normal and pretty pedestrian", maintains Moore. "Our son is now 13. You?ve got to hold your breath and see how they turn out, haven?t you? I?ve tried not to be invasive ? not to sit peering over him reading his texts. But we do a little 'lurking' on Facebook," she confesses with a mock-guilty smile.
"Once there was something horrible on his Facebook page so we asked him to remove it. He was like, 'How did you know?' I said, 'Because we lurk. That?s what we do.' You have to make them ?friend? you, and then lurk,? she instructs, 'or get your babysitter to do it!?
Moore says her biggest extravagance is travel. "We went to Paris with the children for my 50th birthday [last December]. We took a safari in South Africa last year. When I grew up we didn?t take holidays. We didn?t even ask that question. We knew we were just going to stay home and watch television."
Her next film is Game Change. "It involved a tremendous amount of research. My son made fun of me because I played nothing but Sarah Palin speeches and interviews on my iPod," says Moore.
She has not met the controversial Republican politician. "I am never going to be completely comfortable playing a living person who is almost iconic and very present in people?s minds. I hope we pulled it off," says Moore (who is herself an avowed supporter of President Obama).
She will soon be seen opposite Robert de Niro in Another Night (out next year) and Jeff Bridges in a fantasy film, The Seventh Son. She is also writing another children's book, dedicated to her mother, My Mom Is a Foreigner, "about the experience of having a mother from another country ? because so many people do nowadays".
"Freud says you need work and love, and if you don?t have one or the other you?re completely out of balance," says the actress before departing. What?s wonderful, of course, is when you have both. "The best time for me is when I?m with my family, not working but knowing I?m about to do a job."
Does she honestly still worry about getting work? "There are always times when it?s stressful and you feel like, 'I haven?t had a job for a while.' I don?t want to fall into that ditch of not working. So I just try to look straight ahead and hope that the jobs keep coming." You get the sense that even that long-awaited Oscar wouldn?t turn Julianne Moore?s head.
Crazy Stupid Love is out in the UK on September 23.
