Due to be screened on Channel 4 this spring, it follows four young men as they experience the horrors and reality of war for the first time, changed forever by what they see and do.
"I'd never held a gun in my life - now I've got 100 per cent respect for anyone who joins the Army and what they have to go through," says Gerard, one of Britain's brightest young acting talents.
Longtime friends Mark 'Treacle' Tate, played by Kearns, and Shane Gulliver (Matthew McNulty), are both 18-year-old privates serving in Basra in 2003.
It's their first tour of duty and they want to go home and say they did something. But they face complex situations they'd never anticipated and are forced to make instant moral judgements in a brutal, highly-charged and life-threatening environment.
At the end of their tour they return to Britain with trophy photos of incidents in Iraq - and it's the extreme reactions to these that have dramatic consequences.
Gerard adds: "My character had done his training and knows what's expected of him and that's fine, until he has to make difficult choices about what's going on in and around the camp."
It was all very different to life on the Chatsworth Estate as the cast filmed scenes in the searing heat of Tunisia, which substituted for Iraq. "It was very hot when we were running around in all our gear. It made me realise what the soldiers have to put up with. We were carrying Bubble Wrap in our backpacks. They carry all their equipment."
Gerard, 22, did two weeks' basic training in Manchester and Leeds with ex-Army instructors before filming began. "We learned about how the Army works, how to use a rifle, how to carry yourself and look the part. And I also went to see a psychiatrist who deals with post-traumatic stress syndrome, which was useful.
Paratrooper
"We had a paratrooper who had been in the Army for 30 years. He taught us everything we needed to know. If you slipped on something, he'd have you. It really put you in that mentality. We drilled every day. If you didn't carry your weapon right, if you spoke out of line, you were drilled. I can understand how soldiers develop rhino skin. They're really tough. It's amazing how they deal with it - and then come back into normal life."
Gerard also had to deal with a few uninvited guests in Tunisia. "Going to bed with cockroaches at night wasn't great," he laughs. "When the light went on, about six cockroaches would scatter."
And talking of scattering, Shameless fans needn't worry about gay Ian vanishing from the Channel 4 comedy drama in the immediate future. Gerard says he'll be back when the fifth series starts filming on a purpose built set in Wythenshawe at the end of next month. But he's nostalgic for the previous location. "I liked West Gorton. It was more real."
Before that he's playing another role as boxer Terry in Lynda La Plante's The Commander. It means yet more training, this time in the ring. "It's very hard - harder than the Army," he smiles.
Would he ever consider joining up for real? "No, I'll stick to acting. It's a bit safer. But I hung on to my dog tags. I got the whole outfit for a fancy dress but then my mum made me get rid of it because it was just clogging up space in my bedroom.
"I learned so much filming The Mark of Cain. I love programmes that have strong subjects. I like doing stuff I'd be interested in as a viewer. That is important."
"For all the latest news from the world of television, check out Ian Wylie's blog, The Life of Wylie .
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