"He's a nice guy," says Brolin with a grin. "He flies jets now. He's like, 'Yeah, I'm over the climbing thing'. He has no fingers, no toes, no nose …"

Others weren't quite so lucky. Rob Hall, the New Zealand-born leader of the expedition, died during the incident.

"He was a pioneer," says Australian actor Jason Clarke, who portrays Hall, referring to the way Hall led the way in commercialising Everest climbs, successfully and safely taking paying clients to the summit. "It was the beginning of huge numbers going there."

Both Clarke and director Kormákur met with Hall's wife, Jan. It gave them the chance to hear the recording of Jan's final telephone conversation with her husband, patched through via satellite phone and walkie-talkie, while he was stranded on the mountainside.

"It was a very emotional moment to go through," says Kormákur.

Clarke even went hiking with Jan, learning the songs she and Rob would sing to each other; it was an experience that made him determined to get his portrayal right. "I want to do due respect to his legacy as a father and the man, [to] who he is, even to New Zealand."

But meeting Hall's family wasn't enough; Clarke, like several other cast members, went mountain climbing for real. After tackling Scotland's highest mountain Ben Nevis, Clarke headed to Hall's homeland of New Zealand to climb "a couple of the peaks that Rob did back in the day". He was accompanied by Martin Henderson, who plays Andy Harris, another guide who lost his life, and Guy Cotter, a professional climber, consultant on the movie and a friend and former business partner of Hall's.

Clarke had been trekking through Patagonia and Kazakhstan before, but this experience was rather more "hairy".

He recalls one moment, dangling off a ledge in almost 100km/h wind. "I was literally hanging down and looking over; it was 11,000 feet straight down. I was thinking, 'I'm a f****** actor - what am I doing here?' But then you calm down." He smiles. "It gave me an understanding of what these guys did for a living."

Others that caught the climbing bug included Jake Gyllenhaal, who plays climber Scott Fischer, and Brolin, who tackled some Via ferrata in Switzerland - notably at Murren, which meant traversing everything from sheer rock faces to insane suspension bridges, hanging 760 metres above the ground.

"I'd never been so afraid in my life," Brolin admits. "I almost couldn't do it. I was thinking about helicopter rescues and all that kind of stuff - but I did it."

Indeed, it was "the possibility of making a film as extreme as this one" that attracted Kormákur to the project.

"I come from a country that is pretty extreme," says the 49-year-old Icelandic director. "Man against nature is part of who I am. Therefore it's very natural to me; I've always been drawn to those kinds of stories, to both watch and to tell."

His 2012 film, ,dealt with a fisherman who survives freezing Atlantic temperatures after his trawler sinks. Kormákur has a wry smile when asked about the process of making

"First we went to Nepal - Kathmandu is an extreme city. Then you go out to Lukla, which is the most dangerous airport in the world. When we landed, scouting locations, there was a helicopter that crashed next to us. Then shooting - people were getting altitude sickness. You can't get equipment up there - no motorcycles, nothing. Apart from a helicopter, nothing can get there and they can't land."

When the production relocated to the Dolomites in northern Italy, the weather had dropped to minus 30 degrees Celsius.

"We were evacuated from avalanche dangers every other day," recalls Kormákur. "We had to hire a helicopter to bomb down avalanches to get that out of the way, so yes, the shoot was extreme."

So how did the cast react? According to Brolin, some "freaked out and had a meltdown".

"They're actors," he says, "not climbers."

Kormákur tended to "lose his hearing" when there were complaints, but argues that many of his cast were game. "I think many of us, who go through this profession, have the urge to experience it, not only tell it."

Yet as we're told in the film: "The last word always belongs to the mountain." When the main crew returned to London, the second unit faced real horrors - narrowly avoiding a tragic avalanche that killed 16 people back at Mount Everest in April 2014.

With a cast including Emily Watson (as base camp manager Helen Wilton), Elizabeth Debicki (as the team medic, Caroline Mackenzie), as well as Keira Knightley (as Jan Hall) and Robin Wright (as Beck Weathers' spouse, Peach), it adds up to an impressive ensemble - although even this fine cast appears small next to the majesty of Everest, captured splendidly in 3D by cinematographer Salvatore Totino.

Scripted by William Nicholson and Simon Beaufoy, the narrative cuts through the conflicting accounts of the tragedy - including , by journalist Jon Krakauer, who was part of Hall's group and whose version of events has been questioned by others.

"You can't expect anybody between 7,500 and 9,000 metres up in the air to recall anything correctly," says Brolin. "Your IQ drops about 100 points, if you have any IQ at all."

The way Kormákur has directed the film, it's up to the audience to decide who - or what - was at fault.

"It's not shying away from the conflict, but it's not a police procedural," he says. "I also think the storm has way less to do with this than people think … the storm was the ultimate reason why they couldn't get down, but it wasn't the reason why they got into trouble. They were already in trouble before."

Mother Nature, as Everest shows, can be an unforgiving mistress.

opens on September 17