EastEnders' Alex Ferns on leaving River City and his Lennoxtown thriller

The character has wreaked terror on Shieldinch, be it ordering a murderous drive-by shooting or attempting to sex traffic his former fling. This is a guy with no scruples. Harper would stab you in the back while shaking your hand. He would sell his own granny without blinking.

Talk about a bad yin? "He's a lunatic," attests Ferns, as he lounges on the sofa in an upmarket Glasgow hotel. "You just don't know what he is going to do next. He is an absolute psycho."

For his latest trick, Harper has kidnapped three women and is holding them hostage. Events will reach a dramatic climax in Tuesday's episode with Ferns set to exit the show in explosive fashion (no spoilers, sorry).

Today there is no sign of the thousand-yard stare that has become a trademark of his television alter ego. Ferns is a handsome bear of a man with twinkling eyes that dance mischievously above a bushy, salt and pepper beard.

A natural raconteur, he admits to mixed emotions about his departure from River City: delighted to have been given such a meaty story arc, yet vexed about leaving the cast he joined last February.

"This is one of the best experiences I've had in terms of everybody getting on," he says. "There were no egos. It was a breeze. You want to go into work when it is like that compared to some other shows I have been on. When they clapped me off on my last episode, I was a bit sad."

In particular, Ferns has forged a firm friendship with Jordan Young, who plays his cousin and partner-in-crime-turned-enemy Alex Murdoch. This is a bromance, it transpires, born out of a shared love of football, ribbing each other on Twitter and Young routinely scaring the bejesus out of Ferns.

"We were in dressing rooms next to each other and pranked each all the time," he says. "Jordan is terrible for scaring people. He was always waiting for me round corners and jumping out, giving me a heart attack. He got me every time because he knows I'm a pussy cat in real life.

"I did everything to try to get him back, but I just couldn't. He's too clever. Once I hid around the corner when we were due to go up for the same scene. The third AD [assistant director] came to get him to go on set, but Jordan said: 'Why isn't Alex coming out of his room?'

"He knew I was hiding so when I jumped out Jordan just stood there and looked at me." Ferns mimics Young nonchalantly folding his arms, eyebrow arched and looking unimpressed.

"One of my plans was to hide in the back of his car and when he was on the M8 come up from behind him and be like: 'Ya bastard!' but then I thought: 'No. We probably wouldn't come back from that one.'"

He unleashes a raucous, throaty laugh that it is impossible not to join in with.

His time on River City may have drawn to a close, but Ferns is unlikely to be idle. The 49-year-old is penning a drama series called The Hollow Girl which will be set around Lennoxtown in East Dunbartonshire where he spent his early childhood.

"It is about Satanic voodoo and alcoholic cops who are addicted to porn," says Ferns.

The main character, he adds, is a senior police detective who (mirroring Ferns) is 49, soon to turn 50. "This is the strange thing. I found myself writing for a woman. I was writing through her eyes," he muses. "She is funny as hell and doesn't give a s*** what people think.

"The body of a young girl turns up and they don't know who this child is. The detective is struggling with her own demons and the two guys who work under her are both f***** in their own ways.

"It is like True Detective, but centred around Lennoxtown and the Campsies with spirituality, voodoo, Satanism and weird s*** going on. There is this dark undercurrent throughout that whole area which I felt as a child because I grew up there."

The Hollow Girl contains a smattering of semi-biographical detail. "Back in the summer of 78 in Lennoxtown all these pentagrams suddenly appeared on walls and one of the churches was desecrated," recalls Ferns. "Cats and dogs were found with their guts slit open. It was weird.

"It was only one summer. I was 10. I can remember it so clearly. I was terrified. It was quite exhilarating as well. Everyone was wondering: 'Who is doing this?' But then it just stopped."

There are echoes of those memories in his writing. Ferns also paid a handful of incognito visits to his former hometown last year. "I have been driving around a lot without people having known I have been up there," he says. "I've gone to various places and found that quite inspiring."

He envisions a six-part series with the first three episodes already written. When can we see it on TV? "It hasn't been commissioned, but I have had a lot of interest in it," he says, carefully. "I've been given a bit of seed money which has helped me carry on writing it."

The eldest of three children, Ferns swapped Scotland for South Africa when he was 11. "We moved to a place called Secunda in the middle of nowhere," he says. "My dad was an electrician and they were building a big power plant there."

It proved a major culture shock. At the time, Ferns had been due to start high school. "Then the next thing I know I got put back a year and I'm in f****** primary school again – an Afrikaans primary school where very few people spoke English," he says.

"I was getting the f****** s*** beat out of me on a daily basis. For a long time. They would call us 'Englishman' and that kind of stuff.

"It was an all-white school. This was the days of apartheid. Everything was segregated. There was loads of down-at-heel Afrikaners. They walked to school in bare feet. We wore shoes and they would ask: 'Why are you wearing shoes?' We were in a very poor white area."

Tensions simmered. Not least because his family openly mixed with black people. "My dad was a big leftie," says Ferns. "He was a troublemaker. We always had black people around the music shop where my mum worked. My dad used to go into the townships.

"We basically had a cop car parked outside our house all day long checking us out. Although we were never pulled in."

Ferns remembers scrawling "Free Nelson Mandela!" on his school bag. He went off to play rugby and returned to find the satchel gone from outside his classroom. "There was a note saying: 'Headmaster's office,'" he says. "You used to get caned in those days. I got six of the best."

On his hands? "On my a***. I still had my shorts on because I had just been playing rugby. I asked if I could put my trousers on before he caned me, but the headmaster said: 'No.' He put me over the desk and f****** caned me. He hit my legs and cut them open.

"When I got home my mum went straight up to the school. Typical Glaswegian: 'You touch my son again like that, I will batter you.'"

After leaving school, Ferns did National Service with the South African army serving in Angola between 1987 to 1989. He went on to study drama in Cape Town before moving to London in his late twenties to pursue acting.

Yet, what had seemed a clear-cut, linear path, suddenly became fuzzy around the edges. "Cut to 1997, I had just moved back to the UK and got married," he says. "I was walking down the high street in Kilburn, next thing I know, I was f****** cowering on the side of the street.

"Everything felt so loud. I had no idea what was going on. I didn't realise then it was the beginning of PTSD [post-traumatic stress disorder]."

Ferns casts his mind back to his National Service. "I was in Angola for a year-and-a-half getting my ass shot off with the South African army. I was 18 and a soldier. I have to say, it was a buzz. But the buzz is there – and then it isn't.

"One minute you are in the middle of the s*** and the next minute you are sitting back at the base having a barbecue and a few beers. It is that on/off thing." Did he have some close calls in Angola? "Oh, yeah. But I'm still here and that is incredible."

He spent five years in therapy. "My dad was killed in a car accident the next year – 1998 – and so that really took me back a step," he says.

Ferns was doing Romeo and Juliet with the Royal Shakespeare Company in Oban when his father Robert died. Only a year earlier Ferns and his wife Jennifer had got married in the Argyll town. The bitter irony is not lost on him.

The accident happened thousands of miles away in South Africa. "He lost control of his car and broke his neck," says Ferns. "He was 49. My age. When I look back he seemed so old. I didn't really have a great relationship with him. It was tough. But I still loved him."

His period in therapy also coincided with Ferns landing the part as Trevor in EastEnders in 2000. "While I was doing that I was in therapy," he says. "Thank f****** God I was in therapy."

The tale of the typecast actor is well-known, but Ferns could write the definitive book on it. His breakthrough role as a TV soap baddie almost ended his acting career altogether.

He was cast in a ground-breaking EastEnders' storyline that tackled domestic violence. In one jarring scene we saw his character Trevor force wife Little Mo – played by Kacey Ainsworth – to eat her Christmas dinner off the floor.

The powerful portrayal saw Ferns receive death threats from outraged members of the public. When Trevor was killed in a fire in 2002, the actor prepared to move on. Yet in the years that followed the spectre of the role continued to follow him. "I struggled to get seen for any other work after that," says Ferns. "They couldn't see past it."

Ferns recounts starring in Romans alongside Orlando Bloom only to learn later from one of the Shammasian brothers, who directed the film, that when he had first auditioned the casting director had voiced misgivings because of that infamous role as Trevor.

Thankfully, says Ferns, the directors only cared about getting the right actor. "It got me thinking about how many times over the last 15 years that has happened," he adds. "Where someone has gone: 'He's perfect for the role, but …'"

When we speak it is only a few weeks after the sexual assault allegations surfaced against Harvey Weinstein (which the film mogul has denied).

"My take is quite simple," says Ferns. "I have been in the industry 20-odd years. I know that it is there. Everybody in the industry knows it is there. I have heard rumours about various people, including ones that are in the news at the moment.

"For me, it is simple: no is no. If somebody did something like that they need to pay the price. End of story. I don't care if it was 30 years ago. If you did it and that person has been sitting with it all those years because they are too scared to say something because you are too powerful? That is bulls***."

Ferns is based in North Finchley, London, where he lives with Jennifer, 49, an actor-turned-teacher, and their sons Cameron, 15, and Mackenzie, 10. Both boys are keen rugby players, while the youngest also boxes.

Cameron, he says, wants to be a GP. And Mackenzie? "He is either going to be in the SAS or doing 10 to 15 in Barlinnie," says Ferns. "The amount of times we've gone to the school because if anyone says anything to him, he just flattens them …"

With his sons getting older, Ferns has thought about moving north to Scotland or returning to South Africa to deploy the culinary skills which saw him compete in Celebrity MasterChef four years ago.

"My dream is to one day be living back in Cape Town with a burger shack on the beach," he says. "I would be working the grill in shorts and bare feet.

"Great burgers, but just three types: cheeseburger, normal burger and a bacon burger. Great fries and only Coca Cola and ice-cold beer in the fridge. To just live my days like that until I pop my clogs."

Is Jennifer up for that as well? "Oh, f*** no. She would be like: 'Are you crazy? What are you talking about? You would come home smelling of meat every day …'"

His mother Margaret still lives in Cape Town. "She is a psychic medium and very good – she has people all over the world come to see her." Very little gets past her, then? "Absolutely nothing," he smiles.

Does Ferns have a psychic gift too? "I do a bit, but it's something you need to practise. My great-great-great grandfather was burned for being the Brahan Seer – a fortune teller."

One doesn't need a crystal ball to glean that Ferns has plenty more acting adventures ahead. Just don't ask him to critique his on-screen performances.

READ MORE: Our pick of Scotland's best villains of TV and film

"I never watch myself," he says. "It is like a butcher going to watch himself chopping meat on a Saturday. Why would I want to do that? Even watching somebody else act is like a butcher going to watch somebody else chop up meat. It just doesn't interest me."

River City is on BBC One, Tuesday, 8pm. Thanks to ABode Glasgow (abodeglasgow.co.uk)