FRANK Gallagher confounds expectation almost as much as Lenny Murdoch, the character he plays in the BBC Scotland soap opera River City. As fans of the programme will know, gangster Lenny can be sweet as a mince pie one minute while unleashing unprecedented seasonal psychopathy, even upon his own family, the next. (Tomorrow night’s episode will reveal a selection box of Shieldinch residents wishing he were as dead as their Christmas turkey.)

But, you discover, this fascinating duality in the character hasn’t arrived out of nowhere. Even though Gallagher has been playing the angel/Lucifer for a dozen years (at this point most soap actors would simply turn up and say their lines), the actor is still wracking his 54-year-old brain trying to make the character even more convincing.

“Some [River City] writers don’t quite get him, and some think he’s a two-dimensional baddie,” he says with a wry smile. “However, my job is to make him three dimensional. That’s why I always want to explore his head. For example, Lenny has lost two sons. He has murdered his ex-wife. But does he sleep at night? My feeling is he doesn’t get nightmares, because he can justify almost everything he has done, but just maybe he doesn’t sleep too well.”

Gallagher has studied the actions and the motivations of psychopaths. He points out that those who wreck lives seldom look like they have the capacity to do so.

“Jimmy Saville is a good example,” he says. “He was being nice to the paper boy and then he tried to rape him. And what also strikes is that Saville had no real sense of humour. He was disconnected from reality. It was all forced laughter.”

Gallagher wants viewers to (sort of) understand their favourite madman. That’s why he would love to drop in a back story to his character. Who, for example, was the first person Lenny murdered? How did he get involved in crime?

Yet, apart from Gallagher’s astonishing work ethic, another feature strikes home as we chat in a coffee house in the west end of Glasgow. Most actors who work in recurring drama argue they are remote from their screen characters. Not Frank Gallagher. And not only is part of Lenny based on himself, he’s also still prepared to mine his own early life in Coatbridge for character motivation and clarity.

“It’s the sort of thing you would talk to a therapist about,” he says, smiling. “I’m not always conscious of it, but at the same time yes, you know it’s there. As an actor, you can feed off it. When I try to find an emotion as Lenny I try to relate it to myself.”

Young Francis Gallagher had a well of emotional upheaval to draw from. When he was two years old his father died from a brain haemorrhage and his mother struggled to bring up their five children. And just when Gallagher thought life couldn’t get much worse, it did. Asthma and chest-related illnesses saw the schoolboy spend years in and out of hospital. As a result, he grew up detached, physically and emotionally, from the family unit.

Gallagher hardly attended more than two years of secondary school and despite teachers recognising a natural intelligence, he struggled to keep up with classwork. So he became a truant. And when his teenage years arrived so did a taste for Buckfast.

By the age of 16 the youngest Gallagher brother was practising hard to become an alcoholic and was in and out of juvenile court for petty crimes so often he was on first name terms with the clerk of court. Barlinnie Prison was just a couple of miles along the motorway and it beckoned him to reside there for a spell. But then a guardian angel came down from Heaven (well, Cumbernauld) to rescue him, offering an It’s a Wonderful Life changing moment.

“I had a probation officer, David Ramsay, who was also a social worker – a lovely, big, gentle man. And he was exasperated with me. One day he took me aside and said to me: ‘What do you want to do with your life, Frank?’ But I had no idea. Physically I wasn’t very big, so labouring was out. Both my brothers were painters and decorators but I was allergic to paint and it would have played with my asthma.” (It was only when he left home and the family dog behind that he realised the source of his debilitating allergies.)

However, something stirred in the teenager. “I was always an avid reader. I had an insatiable appetite for biographies of actors. And I loved the movies, showbiz. And I loved to sing. I’d sing Jailhouse Rock on the streets.” So he had been suppressing his performance gene? “Yes. I was an attention-seeking wee bastard,” he says. “And at this stage it was desperado stakes. So I said to Ramsay: ‘I’ve always fancied being an actor or a singer.’ And within a couple of weeks I had an audition at the Citz [Citizens Theatre in Glasgow].”

The transformation from tearaway to thespian wasn’t entirely smooth. Social work paid for the teenage Frank to be picked up and taken to drama classes at Cumbernauld Youth Theatre two nights a week (“just to make sure I went”). One Friday however he decided not to go. “I had been having problems. I wasn’t confident. I didn’t even know what improvisation meant. It was only later on I realised it meant showing off. Anyway, I went to my mate’s house and drank Buckie. But Ramsay found me and persuaded me to go back to Cumbernauld one more time. And I did. Half cut. And the drink gave me the confidence to perform.

“The trick then was to go sober, which I managed. And before I knew it I was making my own way to the drama club, taking the bus.”

Others saw potential in Gallagher, such as Daphne Kirkpatrick, a schoolteacher who was part of the drama group. She paid for his audition for the Royal Scottish Academy of Music and Drama. “It was an incredible show of faith,” he says, the note of heartfelt thanks clear in his voice.

Gallagher loved drama college and loved learning. When he left he landed several angry-young-man roles and excelled. But he also showed he could play far more than the bitter, vengeful Glaswegian, appearing in plays by Shaw, Shakespeare, Chekhov. He has since shown a real comedy bent, evidenced in recent stints at Glasgow’s Oran Mor theatre.

Gallagher, however, chose not to go to London, like his actor friends Neil McKinven, Martin McCardie and Paul Higgins. He stayed in Scotland, working in low-paid theatre, at times struggling to put food on the table. Yet the one-time YTS worker is not about to leap aboard the bandwagon that criticises Britain’s privileged actors, the Eton set. “I have no problem with any actor’s background,” he maintains. “And you have to think what sort of life did someone who went to Eton have? For all we know they could have had their arse spanked every morning, or had to warm toilet seats for people. I would say, however, that so much TV drama today seems to cater for posh people, with the working class reduced to being servants again.”

Gallagher, who married his wife Ann in 1990, retains a measure of anger, and when it’s not being channelled via Lenny Murdoch it emerges as a raging political voice. Gallagher is deeply concerned about the erosion of workers' rights, the crushing of unions, the poor being pilloried. “When did people on benefits become scroungers?” he asks. “When the politician’s decided there were votes in it. And now the Tories are stealing the views of the Labour Party. Theresa May is a blatant liar. And she’ll make us pay for Brexit. The SNP are the best of a bad bunch. But as a country we don’t believe in ourselves.”

The tone of voice suggests a dark, unhappy character but Gallagher is anything but. His glass is more than half full. Yet he does admit he needs to leave Lenny behind at times. (The actor returned last week from a break in Toronto and New York.)

“When I play Lenny for 14 weeks [the stints River City records at a time] I give him my everything. He’s always in my head. I don’t want to turn him off. But then I need to get away from him.”

Well, you wouldn’t want to keep a psychopath in your head for too long, would you, even if he is deliciously complex, gallus and full of winning one-liners. And you wouldn’t want to be continually accessing the darker side of your own soul in order to bring out the best, meaning the worst, in your soap character.

“That’s true,” he says, grinning. “But here’s the thing. If I hadn’t been such an arsehole as a kid I wouldn’t have gotten into trouble. And if I hadn’t gotten into trouble I wouldn’t be in the great position I’m in right now.”

River City is on BBC One on Tuesdays at 8pm.

Favourite book

Five Families by Selwyn Raab.

Favourite holiday destination

Niagara, Canada.

Best advice received

Behave yourself.

Worst advice received

I don't think I've had much bad advice in my life.

The moment that changed your life

Turning up at Cumbernauld Theatre in 1982.

Ideal dinner guests

Elvis Presley, Gene Hackman and the missus.

Biggest regret

Life's too short to have regrets.

Career high

Playing Lenny Murdoch in River City for 12 years.

Career low

Many years ago I was in a play called Madam Tee. I'll say no more ...