Inside the head of a hitman ... and under the skin of Superman

Yet Michael Shannon, who plays him in a new film considers Kuklinski, a New Yorker of Polish descent, to be "an unusual man that merited contemplation".

"A lot of people think that he had no remorse or regrets or never questioned his actions. I really don't think that's true," says Shannon, who also stars in the forthcoming Man of Steel, the latest cinematic telling of the Superman tale.

The actor studied 20 hours of interview footage given to him by the film's director, Ariel Vromen, who had sourced an unedited version of a HBO documentary on the contract killer. It enabled him to understand the world from Kuklinski's point of view, if not his compulsion.

"He had a huge amount of rage inside of him based on, I think, a very traumatic childhood. Both his parents were pretty sadistic in their own way and so he had these demons and this job of being a hitman gave him an opportunity to try and do something constructive with them, or at least something for monetary gain.

"But I also think that he was longing for a normal life [he was a husband and father to two daughters whose family never knew what he did]. I think at the end of the day he knew who he was and what he had done and he could just never figure out how to stop it."

"He did this for years and years [his spree is said to have lasted between 1948 and 1986] and nobody came anywhere near him. So, one of the theories we had was that he literally wanted to get caught, that he couldn't handle it anymore. He started getting sloppy because he was burnt out and somewhere in the back of his mind he felt guilty and felt like maybe he should go to prison [where he died, aged 70, in 2006]."

Shannon has won widespread acclaim for his portrayal of Kuklinski – the latest addition in an impressive career that has seen him work with Martin Scorsese on TV series Boardwalk Empire (the recurring role of Agent Van Alden), Sam Mendes on Revolutionary Road, for which he was Oscar nominated, as well as Werner Herzog (Bad Lieutenant/My Son, My Son, What Have Ye Done), Oliver Stone (World Trade Center) and the late Sidney Lumet (Before The Devil Knows You're Dead).

"I just kind of do what people ask me to do," he says. "I don't say no to many things. I usually am pretty excited to get a job, so if someone offers me one I'll take it. But I also don't want to limit myself by saying that I want everything I do to be incredibly different – so if I've already done green and blue I'm never going to do green and blue again. That just seems small minded."

So, what about comedy? Does he hanker to show a goofier side? "I think by and large if you really look at most of what I've done there tends to be humour dispersed throughout, like a little oregano in the soup," he smiles. "It's not like I'm totally joyless. I mean I'm in Bad Boys 2, which is one of the most financially successful comedies ever made!"

Next up for Shannon is Man of Steel, directed by 300's Zack Snyder, in which he'll be playing the main villain, General Zod, a role originally made famous in the Christopher Reeve films by Terence Stamp. Expectations are high, especially since Warner Bros is eyeing it as a possible replacement for its now concluded Dark Knight franchise.

For Shannon, signing up was a no-brainer. "I can't really fathom how someone would say no to Man of Steel," he says. "I kind of thought someone was pulling my leg the whole time until I was actually there, filming, and then I realised they were serious and that they really wanted me to do this. But I didn't have any qualms about doing it."

Did he feel any pressure? "It was a blast. I mean Zack is one of the most fun directors you'll ever work for. It's obviously a very big budget movie with really high expectations but I never felt that pressure on the set. I never felt like, 'oh, I'd better not blow it or I'm never going to work again!'"

The Iceman opens in cinemas on Friday, June 7; Man of Steel follows on Friday, June 14

Film