Jack O'Connell has revealed he did not stay in jail as part of his research for his new film, Starred Up.
The 23-year-old - who portrays convict Eric in David Mackenzie's gripping prison drama, set on location at the former HMP Belfast - admitted he preferred to leave his character once the cameras stopped rolling.
"When they called 'wrap' every night, we'd go back to our lavish apartments. There was that cut-off point," he told Empire.
"But there was one cast member who insisted on staying in the prison, and I really respected that."
The actor continued: "Crumlin Road Gaol is an ancient prison and it's disused now. You'd literally be on your own in a derelict old building."
Jack, who also stars in the 300 sequel Rise Of An Empire, admitted it was challenging making the punches and scrapes look realistic.
"I know David's priority was to make every punch count and resonate with the audience. We didn't want to desensitise any of the violence, so we made every scrap look genuine and not choreographed," he said.
"That was immediately key. You'e not taking huge, glamorous run-ups before throwing a punch, if you know what I mean."
Rupert Friend and Ben Mendelsohn also star in Starred Up, which opens in cinemas on March 28.