On New Year's Eve 2007, when Kate Greene's son Reef was in hospital being treated for cancer, she told her husband Singe she would swap places with the toddler in a heartbeat. The following year, with Reef beating the odds to make a miraculous recovery, Kate found a lump in her breast. By Christmas 2009, she was told she had just 18 months to live, but she died less than a month later, leaving a list of 79 things she wanted her family to do - from looking for four-leafed clovers to diving in the Red Sea.
Kate's story was immortalised in a bestselling book by husband Singe (short for St John), called Mum's List, which is now a film starring Emilia Fox as Kate.
"It's every parent's worst nightmare that your own child will be ill, which is what happened for Kate and Singe when their little boy got cancer," says the actress, 42. "And then you would say, 'I would swap places with my child,' and do anything for them - [but] the horrible truth for her was that she did swap places and got cancer herself and didn't survive.
"For me as a mum, it's incredibly easy to feel the emotions of that..."
Fox, best known for playing forensic pathologist Dr Nikki Alexander in the BBC's long-running series Silent Witness, is a single mum to six-year-old Rose, from a relationship with actor Jeremy Gilley.
"Every parent has to juggle, whether single or together, and deal with different things," says Fox on balancing work and parenthood on her own.
"Obviously, Rose is my priority and I make sure that whatever work I'm doing works for her, and that there's a consistency. That's what I took from my parents: no matter what work they were doing, my school life and family life and feeling very loved and safe was integral to them, and that's what I try to give my daughter."
While filming Mum's List in Clevedon, Somerset, where Kate lived, the actress was also working on the new series of Silent Witness, which starts on January 2. But weekends meant spending time with Rose, which helped her process her emotional response to playing Kate.
"I either had Rose with me [in Somerset] at the weekends or I was driving back to London anyway [to work on] Silent Witness. Of course, the film highlights how important family love is and to make the most of those precious moments you have together. So definitely a way of dealing with those emotions was to go home and make the most of every moment, and be normal."
She describes the role as "a huge honour, but also a huge responsibility" to do Singe and his boys Reef and Finn proud. And while her co-star Rafe Spall could speak directly to Singe to help portray him, Fox had to rely on "Singe taking me through every scene of the film and telling me where Kate was emotionally and physically at each time..."
"We had a good old chat for a couple of hours in a pub and a good old cry when Singe told me the story," she says, "and it was so intense that one of Singe's neighbours saw us and mistakenly thought he was breaking up with a girlfriend!"
Admitting it's the most emotional role she's ever played, "particularly because it's true", she says it really helped that the parents of the boys who played Reef and Finn (Matthew and Will Stagg) allowed her and Spall to become "surrogate parents to the boys for the duration of the film".
"On set, we very much behaved like a family unit. That helped for the emotions of the characters and the naturalness and ease of a family together."
One of the hardest scenes to film was where Kate says goodbye to her sons. "If a mother is preparing to say goodbye to her children, how on earth do you do that? You would want to carry on kissing them forever... It was a tough couple of days on the film."
Also a tough shoot was where Kate reads her list to camera.
"I was very aware that my emotions as Emilia didn't overtake Kate's emotions. I got cross with myself, because it was hard to say the list and not identify with it as a parent myself, and I wanted to keep that in check. I didn't want to cry and make it obvious I was crying on my behalf."
In real life, Kate didn't write her list in one go, but over weeks, with Post-it notes and texts to Singe, who then compiled them all. Its contents ranges from instructions like not letting the boys smoke, always kissing them twice and eating her favourite orange Club biscuits, to more adventurous wishes, including roller-skating around the Natural History Museum (which they've ticked off) and buying a speed boat.
"So many of those things on the list resonated with me," says Fox. "It's a celebration of the very ordinary, normal things in life: you make sure there's food in the fridge, and know what's happening with the afterschool club...
"The notes and the memos and texts Kate left were reassurances for Singe and the boys with that routine to help them through. And [there were] memories which were so personal to her and things she wanted her boys to be able to do.
"What for me is most touching about the list is the way she wanted to remain part of her sons' lives and their future. Even though she was not there physically, she was absolutely omnipresent all through filming."
Making the film has taught Fox the same lesson she got from seeing two autopsies in preparation for Silent Witness.
"One was an older man before I started in the series and later a young man in his 20s, which didn't seem natural as he was in the prime of his life. It was very thought-provoking.
"What I took away from it, by coincidence, was exactly Kate's motto: To make the most of every moment." It's been 20 years since Silent Witness first came to our screens, with Fox joining the cast in 2004.
"I feel very proud of it," she says. "There aren't many shows that make it to 20 years and it's still made with the same passion. We've just shot the most challenging episode I've ever worked on. It has Nikki doing things I've never been asked to do before. It will be a surprise for everyone."
Hailing from an acting family (Emilia's dad is actor Edward Fox and her cousin is Laurence Fox, to name just two), it's no surprise to hear that Rose is "a bit of an entertainer".
"My job is to give her every opportunity and let her choose her career," says Fox, admitting: "I never wanted to be an actress, I didn't have any rose-tinted glasses about the profession because I'd seen the times when my mum and dad weren't working, as well as when they were. There are so many highs and lows.
"Also, when you come from an acting family like mine, it's much more of a challenge to be seen as a person and performer in your own right." She believes her greatest achievement by far is being a mum.
"Motherhood is everything - and it's been the best thing that's ever happened in my life. It makes me feel more complete.
Mum's List is in cinemas now