In Vicious Circle, the new BBC1 film of his extraordinary life, Cahill is confronted by the intimidating figure of Charlie Rice, the local IRA bigwig who demands that he pay the Provos their dues from a recent jewellery theft. Cahill sneers derisively at Rice's threats: "Don't mess with you? Don't mess with me. Do you know who I am?" Underlining the point, Cahill then fixes Rice with a glare that could strip paint at 50 paces. And with that, he goes straight back to tending his beloved homing pigeons.
Ken Stott has just the right screen presence for the role of Cahill. He brings a sense of wordless menace to the character without making him a cardboard cut-out baddie. He makes Cahill a man who cares deeply for his pigeons and at the same time is capable of cold-bloodedly nailing a traitor's hands to the floor.
Plausible ambiguity is the hallmark of all Stott's performances - from the DJ with the double life in Takin' Over the Asylum and the enigmatic detective in Shallow Grave to the craggy coach in The Boxer and the world- weary police officer in The Vice, currently showing on ITV.
The makers of Vicious Circle certainly latched on to that duality in Stott. "Ken doesn't have that classic look, yet a lot of people warm to him," says the director David Blair. "He also has that slight menace. That's important because even though he's not a robust, physical character, he has to get that across."
Talking in Dublin during a break in filming, Stott tells how he aims to make Cahill credible. "With this character, I wanted to pose a question. If you feel sorry for him at the end, then that's a job well done. The interesting part of playing a villain is to find the softer, more gentle aspects of the character. They don't abound in this script, but there are areas in which you can feel some sympathy towards him. If you look at Cahill's childhood, it's very clear that it was terrible. As with most villains, the formative years are very important. Cahill suffered very badly at the hands of the authorities, so it was obvious that he was going to have a lifetime of kicking back - and he did so spectacularly."
With a CV boasting many more hard cases than cuddly fluff-balls, it is clear that Stott has always been drawn to such gritty work. "I'm not attracted to something like Four Weddings and a Funeral," he snorts, "or however many weddings there were. There were too many weddings, anyway; I'd have watched four funerals, though. If it's just pure entertainment, then I don't really want to do it."
Stott has made his name as a readily identifiable geezer from down your street. When people watch him on screen, they think: "Oh yeah, I know that guy. He drinks in our pub." He relishes the sort of down-to-earth, anti-glamour roles that more image-obsessed actors would run a mile from. "You should never be afraid to be ugly," he says. "If vanity creeps in, then you're sunk. If you don't wish to be seen as ugly, you miss out and start playing attitudes as opposed to characters. Emotion is ugly, but emotion is, after all, what we want to see. I don't want to shy away from emotion for the sake of making sure I look okay."
Dismissive of the shallowness of the US industry which is only looking to cast "cute asses", Stott is unlikely to be tempted by Tinseltown. Hollywood, he reckons, is overrun with people who "make a career out of glad-handing and gushing and saying how fantastic the meeting was. I don't want any of that stuff."
And after all, he is doing rather well over here, thank you very much.
`Vicious Circle' Tue 10pm BBC1; `The Vice' Mon 9pm ITV