Naoko Mori is no stranger to challenging roles. At the age of 17, she became the first Japanese national to land a lead role in London’s West End, playing Kim in Miss Saigon, and has since portrayed Yoko Ono in BBC drama Lennon Naked.

But her latest finds her playing Japanese climber Yasuko Namba in Everest, Icelandic director Baltasar Kormakur’s real-life recreation of the 1996 mountaineering disaster that claimed the lives of eight people.

It’s a role that saw Mori push herself physically and emotionally to such an extent that she has since described the experience of making the film as life-changing. Yet she wouldn’t have it any other way.

“For me, there’s always a huge attraction to playing real people," she admits. "But with it comes an incredible sense of responsibility because you’re playing a real person in a real event. We just hope we’ve done the story and the people affected justice.”

Getting to know Namba was difficult in itself, given that there is not a vast amount of information available. But a chance encounter proved invaluable and even felt like fate working in strange ways.

“We were in Nepal, on the Everest trek, and had reached this horrifically terrifying suspension bridge, when I was suddenly aware of a woman behind me while we were waiting for the cameras to be set up. She seemed very upset at the name ‘Namba’ on my backpack. When I told her why we were there and who I was playing, she replied, 'Actually, she was my friend'. It still gives me goose-bumps now. But having met her and spoken to her, I got a lot of insight into Yasuko.

“She was a 47-year-old businesswoman, who was a very experienced climber – she had been climbing since university and had already completed six of the seven summits in the world, so this would have made her complete. Everest was the seventh summit of the seven, which was also incredible for her because she didn't have a lot of commercial backing or sponsorship. She did it privately. She was quite reserved, but also feisty and determined.

"I think we all have things in our life... demons or things, and I think it [climbing] was her way of dealing with certain things. And it did help her. She loved it so much. She used to get up in the middle of the night and train before going to work, and she would run up and down staircases in skyscrapers during her lunch break to train. She was incredibly committed.”

Talk of training brings us, almost inevitably, to Mori’s own physical preparation for Everest, which she admits was even more rigorous than she thought. She already had a foundation in the fact she is a keen rock climber but nothing could prepare her for the effects of the altitude training.

“Believe me, you have no idea until you experience it, what it does to your body, your mind and your brain. Once we got up there, and reached Namche Bazaar, which is about 3,400 metres, you really feel it.

“There was no way out for any of us. You just had to fight it. And it affects people differently. I didn’t get nausea or headaches, but I did get sleep apnoea, which is where your body stops breathing in the middle of the night, and it then wakes you up and makes sure you start breathing again. That was an interesting experience because while you do get used to it, it means you’re not getting any proper sleep.”

Added to that were the -20 conditions, the lack of hot water and no proper toilets. “I was the only female cast member, so that was a bit nervewracking, hiding behind a yak to pee and making sure no one was watching.”

That said, Mori has nothing but praise for her fellow cast members, most notably Jake Gyllenhaal and Josh Brolin, and the way they bonded, adding: “I was in heaven anyway because there I was in this sea of bearded, burly men and eating Italian food – which has always been my favourite.”

A fan of both actors, it was nevertheless Brolin that left the most indelible impression upon her, not only for his sense of humour, but also for his gallantry.

“Josh is a true life hero," she says. "There was one take, when we were in the middle of this suspension bridge, when one of the yaks panicked and started a stampede towards us. And this was a narrow bridge with literally nowhere to run. So we got kicked and head-butted. I went down. But he jumped on top of me and covered me from all the other yaks.”

If such an experience provided a salient reminder of how dangerous mountaineering can be, then news of the 2015 Nepal earthquake and subsequent avalanches on Everest, which resulted in 18 deaths, came as a horrifying shock, not least because the second unit of the film were still on location there.

“We were just absolutely gobsmacked. Kent Harvey, our director of photography, was still acclimatising at base camp, and we lost some of the sherpas we’d worked with. The producers of Everest emailed us and let us know they were fine straight away.

“But it serves as a reminder that it’s such a treacherous place. If you go to the official website for Everest, we have a charity link to Oxfam that encourages people to give in support of the relief that is still needed there. And hopefully, if people see the film, they can chip in because they still need our support and help.”

Everest opens in cinemas on September 18