Ad Astra was "the most difficult thing" Brad Pitt has ever worked on.

In the sci-fi drama, the 55-year-old plays astronaut Roy McBride, who goes on a life-threatening mission to find his long-lost father Clifford, played by Tommy Lee Jones, who went missing during an expedition searching for signs of alien life.

Director James Gray had his leading man suspended from zero-gravity rigs for hours at a time during the lengthy shoot, and Pitt confessed that it was hard work.

“Cracking this was the most difficult thing I have ever been a part of,” he told British newspaper The Telegraph.

“Actors require each other, or at least some kind of environmental stimuli, to do their best work,” Gray said. “And when you’re hanging upside down on trapezes for 12 hours, there’s very little of that.”

The Oscar-nominated actor also revealed that the slightest false note while recording his voiceover for scenes set in space would “tip over the whole cart”.

“The minute you think you know everything about film-making, you’re done," he added.

Elsewhere in the joint interview with filmmaker Gray, the Moneyball star discussed his frustration at playing Heinrich Harrer in Jean-Jacques Annaud’s 1997 biopic, Seven Years in Tibet.

The movie was adapted from Harrer’s memoir, which described his seemingly inspirational life after escaping from a British prisoner-of-war camp in India in 1944.

However, it missed out one of the most important details of Harrer's story, which was the fact he had also been a sergeant in Adolf Hitler’s SS.

"It was only halfway through the film that I found out that my guy had been a f**kin’ Nazi. I think ignoring that was a mistake in the film,” Pitt fired. "I mean, to have come from that – it’s even more incredible for him to have made this spiritual flip. So why hide it?”