Drew Barrymore, Toni Collette and Paloma Faith walked the red carpet at the European Premiere of Miss You Already in Leicester Square, held in aid of Breast Cancer Now.
Barrymore stars as Jess, who lives in a boathouse with her boyfriend Jago (Paddy Considine) and desperately wants a baby.
Her best friend since childhood, Milly (Colette), is a successful career woman with a rock-star husband Kit (Dominic Cooper), and two children.
But when Milly is diagnosed with breast cancer she needs Jess's support more than ever - and when Milly becomes pregnant, their bond is tested to the limit.
Collette, 42, said: "We did not want to misrepresent the experience in any way way, shape or form, so we did a very thorough amount of research and spent time with lots of different people who'd had cancer or worked with people who'd had cancer.
"I've had friends who have passed away, and friends who are still here, but it's such a personal experience and journey - and I learnt much, much more from doing this film."
The actress, who wore a stunning green Herve Leger dress, added that she'd had the "time of her life" working with her co-star - while Barrymore praised "the chemistry" she felt with Collette.
Collette sported a short crop, having shaved her head to play the chemotherapy patient.
Jacqueline Bisset, who plays her on-screen mother Miranda, reported: "She was very brave. The scene where she did have to have her hair shaved off - I must say I did baulk slightly, as her 'mother'. But I was very admiring that she did it with such a relaxed manner."
Considine, 41, explained that it was important to explore issues that viewers will relate to and connect with.
He said: "Isn't that why we do it? If you're not telling stories that give some kind of enlightenment in some way, then I don't think it's worth doing, really."
Catherine Hardwicke, 59, who directed the film said: "We hope that it does encourage people to love their best friend more, support their friend. Add laughter to their lives even if they're going through a tough time."
Singer Paloma Faith, who contributed an original song to the film titled The Crazy Ones, was on hand to sign autographs for fans.
Breast Cancer Now is a medical research charity which aims to end deaths from the disease by 2050. The charity, whose patron is The Prince of Wales, became the UK's largest breast cancer charity after a recent merger between Breast Cancer Campaign and Breakthrough Breast Cancer.
:: Miss You Already will be released in cinemas on Friday September 25.