The film is an adaptation of the miraculous 2018 Tham Luang cave-diving rescue mission in which a football team was rescued from a cave in Thailand.
As part of the film, all cast members did their own stunts, so there were long hours of underwater training required.
Taking on a lead role in the film, Farrell told Variety: “The water stuff was horrible!
“They built four or five tunnels that were designed based on the drawings of the Tham Luang cave.”
The Dubliner admitted things got intense during filming.
“I mean, there were twists, turns, down, up, pitch points, stuff you had to go upside down. And we had safety divers, but it’s still water.”
“And there was no f**kin’ surface, they had holes on the side of the tunnel where the safety divers could watch us,” he explained.
“Yeah, I definitely had a few anxiety attacks under the water.”
Bateman echoed Farrell’s panicked response to the intensive shoot, claiming that he didn’t expect the process to be so realistic and gruelling.
His co-star Tom Bateman also confessed he suffered panic attacks.
“I didn’t realise they had one of the most incredible camera operators who could be in those tunnels with us.”
“The dude was, like, going backwards and stuff! So yeah, I had a hell of a lot of panic attacks in there.”
The real-life cave divers, Rick Stanton, Vernon Unsworth, Connor Roe and Josh Bratchley, who rescued 13 people from the caves consulted on the film, giving actors an account of their perspective.
“Ultimately, we all go cave diving for fun,” Connor Roe said.
“And we’re all part of rescue teams to give back to our communities. And so we never expect anything from that, it’s just a nice thing to be able to do. So to find ourselves in this position, it’s nice to share this story with the world.”
The four British cave divers rescued 12 boys and their coach after they were trapped 2.5 miles inside a flooded cave for 18 days following a flash flood.
Directed by Ron Howard, the film also star Viggo Mortensen and Joel Edgerton.
Thirteen Lives will be available on Amazon Prime from August 5th.