Martin Compston: “Whoever thought this would happen to me?”

It is morning in Los Angeles when Martin Compston calls. The sun is out. He has taken the dog for a walk. The previous night, Irvine Welsh came round for a steak dinner and to deliver an advance copy of his new book. With a June wedding back in his native Scotland around the corner, life is looking peachy.

Then there is the return of Line of Duty on BBC Two, to fill the Happy Valley-shaped hole in our lives for top-class crime drama. Compston plays DS Steve Arnott, softly spoken lothario and committed police officer for the AC12 anti-corruption unit. The first series featured Lennie James as a charismatic officer whose impressive stats masked his dodgy methods. Then Keeley Hawes delivered a career-changing performance as DI Lindsay Denton in a second series that gripped the nation. How do they top that?

“Last year was Jed’s [Mercurio, Line of Duty creator] masterpiece,” says Compston. “And I know it is annoying when bands come out and say their new album is the best one yet but I think Jed has surpassed himself. Again.”

In the third series, Daniel Mays plays the aggressive brute leading an armed response unit, whose itchy trigger finger brings him into AC12’s crosshairs. “Danny is incredible. We rely on him to set up the show, give us something to investigate. The audience has to be intrigued by this guy.

Danny is incredible. We rely on him to set up the show, give us something to investigate

“Other shows, like Luther, have maverick cops who don’t pay attention to the rules. Our thing is that everything we do has to be by the book. So last year’s ending wasn’t all grenades and car chases. It was us catching Keeley with good police work.”

Compston, 31, could have been coming to the end of a professional football career. But, after playing for Morton in the Scottish First Division as a teenager, he scored the lead role in Ken Loach’s Sweet Sixteen back in 2002 – unknown, untrained but perfect as troubled teen Liam.

“The future was bright, I was very happy playing football,” he recalls. “But my teacher encouraged me to audition for Sweet Sixteen. The scenarios we were given was everything I knew. If you hung around after school, you knew these characters who have nothing to lose and see drugs as their best way to have a nice car and a bit of respect locally.”

The transition from talented teenager to working actor was not straightforward. Compston almost quit after one audition. “I went down to London with my best ‘night out’ shirt on. Everyone else is dressed all casual, and the guy from Greenock looks like he is going to a nightclub. I felt so out of place. They gave me the wrong speech to learn. I spent three days learning my audition piece, and then had five minutes to learn something new.”

He got the role (in ITV’s The Royal). Monarch of the Glen followed. “Six months a year, hitting your mark, camera angles, different directors, lots of great actors. That was my drama school.”

An outstanding performance in Red Road confirmed his talent, then came The Disappearance of Alice Creed and a star turn in Filth. So has he had clashes with the classically trained, Etonian or Rada types?

I was thinking, I will watch him in action and learn his secret. The secret is he is just that good

“There are clashes of styles all the time. But the biggest was with Peter Mullan. I need to get into the right frame of mind. But Peter has this annoying ability. He will be chatting and laughing away, right up to, ‘Action’. Then, bang. Action. He is in it. I was thinking, I will watch him in action and learn his secret. The secret is he is just that good.”

Compston’s secret? Aside from ability, it could be his work ethic. For Line of Duty he spends months living in DS Arnott’s English accent, only breaking it when his fiancée, Tiana Chanel Flynn, visits. “You don’t feel yourself talking in a different accent,” says Compston.

Filming will begin on series four of Line of Duty after his honeymoon. “It has been solid hard work. But that original audition gave me a chance to pursue a life I never thought I would have. I’m talking to you from my home in LA. Whoever thought that would happen to me?”

Series three of Line of Duty is on Thursdays on BBC Two