The two stars play Scott Fischer and Beck Weathers in Icelandic filmmaker Baltasar Kormakur's new epic Everest and both were left a little 'loopy' after taking their research too far.

Gyllenhaal says, "We stayed in there (simulator) too long because we were enjoying ourselves. We were between 26,000 and 36,000 feet... Three days after that, we were in a severe depression."

Brolin tells Outside magazine, "I remember in rehearsals, I was like, 'I don't feel right, do you feel right?'"

But Gyllenhaal admits the experience was helpful - because it made him realise how the brain works at altitude, where the air is thin and survival is key.

He tells the publication, "They (real-life characters) were having to make moral choices at 28,000 feet, and area where the body does not function."

The film is loosely based on author Jon Krakauer's account of two rival Mount Everest expeditions, led by mountain gear outfit store bosses Fischer and Rob Hall. Both teams, featuring experts and amateur climbers, were left stranded in a storm at 28,000 feet and eight people lost their lives in a day.

The film will open the Venice International Film Festival in Italy on 2 September (15).