As yet unwed, and childless, the 36-year-old actress smiles, turns those bright cornflower-blue eyes up to full wattage, and prepares for battle. "I think it's only normal for people to ask my views on motherhood, especially of someone my age, because it seems like the obvious thing, like 'Why haven't you done it yet?' But it doesn't bother me. It's not the cross I bear. I simply have no idea at this stage in my life. Besides, I'm still young," she says.
"I have one nephew and four nieces, I've seen three births, two vaginal, one C-section... I've been there for the dirty diapers, I've done the bottles... I know what it takes to do it. I think that I have a good sense of what it takes to be a mother and, although I'm not mother, I feel that it's the closest I can get.
"I also understand about loving very deeply and knowing that I would do anything just to save somebody that I love. No, I'm not the mother of a sick child but I can imagine those feelings.
"I also don't know what it's like to be an 18th-century pickpocket, you know what I mean?" she says, in reference to Gangs of New York. "Or to have a mother who killed herself as with In Her Shoes, and have to deal with that. But that's what us actors do," she says.
My Sister's Keeper is pure Oscar fodder – Diaz plays a mother who has a genetically engineered test-tube baby to use, essentially, for spare parts for her eldest daughter who is dying of leukaemia. Sofia Vassilieva gives a heart-wrenching performance as a sick teen whose illness is only complicated by her fighting family. Abigail Breslin, already a former Oscar nominee for Little Miss Sunshine, plays the sister whose burden it is to help keep her big sister alive.
Asking an actor if they would like an Oscar is usually like asking a dog if it would like a bone. However, in Diaz's case, her reply is unexpected: "No! Do you know what you have to do to get an Oscar? You have to talk to a lot of you people for like days and months on end. Not you, though. I'll talk to you any day!" she adds, realising that this might not be the wisest remark.
But her response is only partly in jest. The truth is that she is one of the most "papped" actresses, owing to her past relationships with Justin Timberlake, Matt Dillon, Jared Leto, John Mayer and, more recently, British model Paul Sculfor.
High-profile legal battles with US tabloid media over paparazzi photos have left her cautious: "Look, when I meet people I'm happy to say hello. I'm not in a bubble; and I don't go through the world not talking to people. I'm in the world; it's just the way I live my life. But I can't always stop and take pictures or sign autographs because everybody's got a camera these days."
The paparazzi, it seems, won't leave her alone – even suggesting she had shaved her head for a poignant scene in her latest film. "Of course I didn't shave my head! I mean, I would be like a scientific freak right now. They would have me in a lab with my scalp, trying to figure out how I grew my hair back so fast. But the paparazzi will say anything to sell their pictures, believe me," she says, playing with her wispy blonde hair, as we sit together in a Santa Monica hotel room.
One of Hollywood's most successful models-turned-actresses, by her own admission she has no desire to churn out films. She has made only 25 movies – not including her voice work in the Shrek animated series – since she first appeared 15 years ago alongside Jim Carrey in The Mask.
"But awards have never been a motivation to do what I do. I'm not doing this to be 'on top' or whatever. I'm doing this to tell stories, particularly with this one. So when I accepted this role, I wasn't thinking to myself, 'Oh people don't see me as a mother! I'm gonna play a mother and then maybe I'm gonna get these other parts...'," she says, adopting an over-the-top "lovey" voice.
"And it's not like I even view this role as some kind of stretch because it feels as if I haven't played the same character twice or even told the same story. I look at it sort of like, 'OK, I did The Mask, and then I did Last Supper, Feeling Minnesota, My Best Friend's Wedding... and then There's Something About Mary that made everybody look. But then I did Being John Malkovich, Any Given Sunday, Gangs of New York and Vanilla Sky... Oh my God! Why am I going through my career?!" she laughs, as if waking from a personal reverie. "Umm, you know, so I feel like I have done a lot of different things."
It was Diaz's demonstrated versatility which attracted My Sister's Keeper director Nick Cassavetes in the first place: "I've been a fan of Cameron for a long time and, frankly, her dramatic roles are among my favourites. And I understand that she is most often associated with comedy because she's so good at making people laugh. I mean, I wouldn't want Sacha Baron Cohen to make me cry but I think Cameron is equally capable of doing both," he says.
Submerged in her role as a mother forced to watch her family fall apart, Diaz sadly experienced her own loss in the midst of filming My Sister's Keeper. Production came to an abrupt halt last year when her 58-year-old father Emilio Diaz suddenly died of pneumonia. "I was in shock," she whispers. "My dad was a huge part of my life and I think about him every day still. He will be a part of me forever."
However, it's clear that Diaz doesn't like to be sad for too long. She has an infectious laugh and nobody could accuse her of not having a sense of humour. She sent up her former relationship with Timberlake – nine years her junior – in a sketch on US satirical show Saturday Night Live entitled "The Cougars' Den". Co-starring Alec Baldwin, she portrayed a particular lustful "cougar" known as Kiki Damore. "I like to enjoy myself," she says. "If you can't laugh at yourself, then what can you laugh about?"
Together with Leonardo DiCaprio, Diaz was one of Hollywood's first "green advocates", both driving Priuses and appearing on platforms promoting sustainable living. And, proving that it's not just lip-service, today she sips from a huge aluminum water bottle and a thermal coffee mug.
"What's wonderful right now is that there's more and more opportunities to incorporate this sustainability into your life," she says, eagerly indicating her one-and-a-half-litre water bottle. "I have 10 of these at home, and then just fill one or two with water before I leave for the day and keep them with me. I think right now is a critical time; it's time for us to learn more and I want to be part of helping people to learn more.
"I can't always be perfect but you try to balance it out; it's not a perfect world. Unfortunately, I can't transport myself to a world where we can do everything perfectly all the time but every day we're moving forward to create that world and that's what I'm trying to do."
'My Sister's Keeper' opens on 26 June
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