Cameron Diaz: "You cannot make the decision to let a child die"

Following roles in comedies such as Charlie's Angels and last year's What Happens In Vegas, she admits that it was new territory acting in the big-screen adaptation of Jodi Picoult's heart-wrenching novel My Sister's Keeper.

Reflecting on her portrayal of mother Sara Fitzgerald, whose child is dying from leukaemia, she explains that she had to grow into the role.

''There was a certain level of maturity that had to come with playing this role, that I think I had to step up to, which I was happy and thankful for,'' she says, picking her words carefully.

In the film, Fitzgerald chooses to have a second child by IVF as a blood and marrow donor for her first daughter Kate.

The movie co-stars 13-year-old Abigail Breslin, best known for her Oscar-nominated turn in 2007's Little Miss Sunshine, as the youngest daughter Anna, who sues her parents for the medical rights to her own body, when she's expected to donate a kidney to her sister Kate (Sofia Vassilieva).

Cameron, 36 and single after a string of boyfriends including Justin Timberlake, admits it was a ''touchy'' role to take on, but she felt ready for it: ''It was a nice stepping stone in the experience of life.''

Casting Cameron as a mother of three teenage kids is an unusual choice for any director, but Nick Cassevetes, who won critical acclaim for 2004's The Notebook, thought otherwise.

He picked her precisely because she had done so little drama, saying: ''I really didn't want to cast an actress who had done something similar for this film.''

And Cameron hasn't let him down. She speaks so passionately about her character Sara, it's easy to see she put a lot of herself into the film.

''I feel like Sara is a warrior,'' she gushes. ''She doesn't have a moment not to be vigilant. She can't be apathetic one second of the day.

''She has a child who's dying and she has been through a decade of this vigilance. So, she's just sort of pushing through this.

''Even when the child is in remission, there's no time. You can't just go, 'Oh, well, now I can breathe', because at any moment there might be a 103 temperature and nose-bleeds, and it all begins again.

''But she has such an intense focus that there wasn't really any room for anything else in her life. And I found that made it easier for me to just keep going.''

To research the role, Cameron met mothers with sick children who were going through the same experience as Sara.

''No matter what the circumstances were - everybody had a different financial position or some were married and some were divorced - they all said that there's just not a moment that you aren't focused on keeping your child alive,'' she says.

''Whether it's reading the charts to make sure that the doctors are doing their job, or thinking, 'If this treatment doesn't work, what's the next treatment?'.

''All these responsibilities the mothers take on, and everybody else in the family suffers. This is the thing with families who have a member who has special needs, everybody else's needs go to the wayside.

''What's so wonderful with this film is that you see how it's not one person's story - it's not just a mother losing a daughter, it's a sister losing a sister or a brother losing a sister, a family losing their family together, all of them suffering separately as well as together.''

The film explores the feelings of each family member and the complex relationships between them - and, of course, how we cope with the prospect of death.

''For Sara, what she is fighting against is the inevitable. Death is certain and unchangeable, but what Sara was trying to do the entire time is to stop something that is inevitable,'' Cameron says.

''Humanity thinks we can change the outcome of things, by sheer will. We've created technologies to change things. But, ultimately, there are certain truths you cannot change.''

In this age of test-tube babies, the film also throws up the moral quandary of whether it's right to have a child as an organ donor for a sick sibling.

Cameron says she didn't make the film as a ''moral statement'' and that in her discussions with parents, the unanimous answer was that it would be a ''tough decision to make''.

''But, ultimately, you cannot make the decision to let a child die. It goes against every fibre of a parent,'' she adds.

It's pretty heavy stuff and Cameron is clearly quite emotional, the smile's faded, so it's time for a lighter question.

She seems to have ignored the old adage about never working with animals and children. But Cameron doesn't bite. Instead, she's full of praise for her young co-stars.

''First off, Abigail and Sofia are both total pros,'' she says, blue eyes sparkling again.

''These girls are such fine actresses, and it's amazing to work with Abigail. She's so present, she knows exactly what she needs to give. I learned so much from her.''

Sofia, now 16, had to shave her head for the role of Kate, something Cameron admits she couldn't do - she opted for a bald cap instead.

''Oh my God, Sofia was 15 years old. Fifteen! That's like the most formidable years of your teenage life where you get a sense of yourself and your self image. Her long hair was her pride and joy, so I was so inspired when she shaved it off.

''She owned it and, after work, she would have the make-up artist paint these great designs on her bald head and she'd put on these great outfits - it was amazing to watch.''

Cameron gets an equally glowing report from the two girls, who say she took on a ''motherly role'' on set and off set, making snacks for the cast in her trailer.

Sofia sums her up: ''You want to hug her and never let go.''

California-born Cameron grew up on the golden beaches of San Diego and became a model after leaving high school at 16.

She got her first acting break at 21, when she auditioned for The Mask without any formal training. But it was the 1998 comedy There's Something About Mary, alongside Ben Stiller, that would make her name as a comic actress.

Over the next 10 years, she worked her way up to become the highest-earning woman in Hollywood, raking in 15 million dollars (£9.1m) a movie, and one of the most written about.

Her personal life has been widely documented, especially her rocky relationship with pop star Timberlake from 2004 to 2007, and when her father sadly died of pneumonia in April last year, during filming.

Today, she's passionate about her family - and says the film echoes her bond with her older sister Chimene.

''It's one of the most intense bonds, being with somebody from the time you enter the world and being that close to one another.''

She recently said she wouldn't rule out having children of her own one day, but for now she's happy just to be an aunt.

''My sister has four children. And watching her kids with each other is just amazing,'' Cameron says, with a broad smile.

''As you get older, you see things, and you remember when you were a kid and people said, 'Oh, wait until you get older, you will understand'. And then you go, 'Ah! I understand'.''