IT is so cold that Gerard Butler’s bicep is turning blue.

He isn’t complaining but the icy air is starting to affect his breathing. It might be something to do with the short-sleeved T-shirt he’s wearing while everyone around him is bundled up in parkas, furry hats, scarves and gloves.

The gunfight he is filming is meant to take place in sunny Los Angeles but instead he is freezing on the outskirts of Atlanta, Georgia, where it is hammering with rain and threatening to snow.

“I’m from Scotland so my Scottish blood is helping me,” he grimaces. It’s not clear who this is convincing.

“I got here early before the movie started filming and we had an ice storm and the city shut down and I was like, ‘I am so f***ed, I’m going to be in a T-shirt every day’.”

We are on the set of his crime thriller Den of Thieves, in which the Paisley-born star plays a hard-as-nails, heavily tattooed career detective and general alpha dog, known as Big Nick. Thermal underwear and a woolly hat wouldn’t suit the tough-guy persona and he’s pretty deep into character.

“I started dressing like him, I started walking around like him.

“I remember I was out at dinner one night with Christian [Gudegast, the director] and I thought I was eating sushi.

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“Whenever I would do something he would be like, ‘The way you put your hand over that glass, that’s Big Nick. The way you’re eating is Big Nick’.

“And then he says, ‘That’s raw chicken that you’re eating’.

“I thought it was fish! But I was just voracious; Nick has that animal-like tendency, like a T rex, so while I’m talking and eating and I’m just thinking ‘fish’, it turns out I had eaten about a pound of raw chicken.”

“It was tough to get in this head space. I was super-nervous about this movie because it’s a very intense character for me to climb into. I don’t think you fully get rid of it until the end of the movie.

“Ambien [the medication used to help insomnia] helps me a lot. But you also do get used to it and you learn to sleep. You switch off, you read, you watch a movie. I like to meditate.”

Butler may be best known for action films Olympus Has Fallen and London Has Fallen, as well as fantasy epic 300, but he says Den of Thieves is more realistic. “If you compare it to 300, it’s a little more real. It really tries to climb into that vibe of cop life and gang life and criminal life."

Den of Thieves is in UK cinemas now