'The Revenant' cast went to "bootcamp" to prepare them for freezing temperatures on set.
The adventure survival film took nine months to make and the cast even visited Russia to get in the snow, but Domnhall Gleeson - who stars as Captain Andrew Henry alongside Leonardo DiCaprio's Hugh Glass and Tom Hardy's John Fitzgerald - admits nothing could really "prepare" them for the extreme conditions they faced.
Speaking to BANG Showbiz on the red carpet of the film's European premiere in London's Leicester Square, he said: "We did a bootcamp and we thought that would prepare us but actually the weather wasn't so bad in the bootcamp. So it really did nothing you know.
"Just making sure you put on your thermals in the morning was the best preparation you could do. And apart from that it was: keep your feet moving and keep as dry as you can."
On some days the cast and crew worked in below 30 temperatures for 12 hours a day and by the end of it, were fit for nothing but bed.
Asked whether the all-male cast would hang out after filming, the Irish actor said: "No, the boring truth is that we rested. It was so extreme; the conditions, that your body just needed rest.
"So everybody would say a sad, quiet good night and shuffle off to their rooms. It was boring, but that was the truth of it."