Director Tom Harper and Eddie Redmayne went through a physically grueling experience while filming their new adventure movie The Aeronauts. 

The English director — who is known for his work on Peaky Blinders and Wild Rose — explained to uInterview exclusively how he and the Oscar-winning actor underwent hypoxia training at a military training base in the United Kingdom in order to prepare for the film, which focuses on a pilot and a meteorologist’s fight to survive while seeking to make historic discoveries in a gas balloon in 1862. There, they entered a decompression chamber to simulate the experience of being at an altitude of roughly 30,000 feet and were “starved” of oxygen.

Harper said both he and Redmayne wanted to know with as much accuracy as possible what it would have been like to be deprived of oxygen under such circumstances during the mid-19th century. The director revealed parts of the movie were filmed in a helicopter and said a stuntwoman was present on set, and also said principal photography for clouds at high altitudes was made at various locations, including New Orleans and South Africa.

“Right from the very beginning, we really wanted to make the film as realistic as it possibly could be in terms of the flight and how it looked and the visceral feeling of what it was like to be in that balloon,” said Harper before admitting that the studio was cooled down to -1 degree Celsius as part of the shooting process. “That led us to building our own period-accurate, 19th-century gas balloon.”

The director also confessed he and Redmayne — who plays real-life meteorologist and astronomer James Glaisher in the film — had vastly different reactions to their experience in the decompression chamber.

“Hypoxia apparently affects everybody differently,” said Harper. “In my case, it made me feel rather sick and lose my memory, but in Eddie’s case he became rather fervent and passionate and quite euphoric.”

Redmayne wasn’t the only one who enthusiastically approached the physical aspect of filming The Aeronauts, however. Harper revealed Oscar nominee Felicity Jones — who plays pilot Amelia Wren in the movie — was also “adamant” from the start about taking on the challenges associated with low oxygen levels at high altitudes. The director said the scene where Jones re-enters the gas balloon was one that particularly stuck with him.

“That scene shows a sort of beautiful intertwining of the physical and the emotional, and it’s really something to behold,” said Harper. “[Felicity] does something quite exceptional there.”

The Aeronauts also stars Himesh Patel, Tom Courtenay and Phoebe Fox and will be released on Friday (Dec. 6).