Line of Duty's Martin Compston on playing DS Arnott, cliffhangers and acting in a new Kiera Knightley film

Martin Compston – Detective Sergeant Steve Arnott in the BBC’s drama of the moment, Line of Duty – descends from the London hotel room where he’s been recovering from jet lag. We’ve already had to re-arrange once because he had been too knackered to string coherent sentences together, having just completed a fortnight of night-shoots in Prague for a Keira Knightley movie (more of which later), made a quick trip home to Los Angeles to see his American wife, Tianna Chanel Flynn (also more of whom later), and found time to nip up to Scotland to see his parents.

There are faint vestiges of weariness as we settle down in the hotel bar, but generally the 32-year-old actor seems chipper enough. Does he live full time in LA now?

“That’s not what the wife and the dog think”, he says. “I’ve spent maybe three weeks there since last May. We got married in June, then filming in Scotland in June, July and August, and then filming in Belfast till Christmas, and then I started in Prague on the fourth of January. I’m under no illusions about how lucky I am at the moment with work going so well, but it’s not been the most ideal start to a marriage.”

It’s the Belfast leg of this hectic schedule that is the reason for our meeting. Jed Mercurio’s award-winning drama about a police anti-corruption unit, AC-12, is filmed in the city and is now in its fourth series, having been promoted from BBC2 to BBC1. “It’s probably the series I’m most nervous of”, says Compston. “I was very happy on BBC2 and there’s no doubt that it [being on BBC1] opens you up to more viewers and stuff, but there’s more pressure on us.”

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Compston, along with Vicky McClure and Adrian Dunbar, are the core of AC-12, investigating (in series one) the head of a serious crime squad (played by Lennie James) with a suspiciously high clean-up rate. Keeley Hawes joined the cast in series 2 as a compromised officer whose information led to the ambush on a police convoy and death of three colleagues, while Daniel Mays starred as a trigger-happy firearms officer in series three.

But the real villain of a storyline that spanned all three series was DS Matthew “Dot” Cottan – aka “The Caddy” (an insidiously menacing performance by Craig Parkinson), AC-12’s very own bad apple. Before finally taking a bullet in the last episode, Cottan had nearly managed to falsely finger Compston’s DS Arnott as the murderous “Caddy”.

“I remember talking to Craig after we’d been given the scripts for the first three episodes, and I said to him “Me or you is going to die at the end of the series”, says Compston. “I said, ‘One of us is going to bite the bullet’. Jed said that I did die in one of his drafts... it was that close.”

The new series of Line of Duty has managed a casting coup in Westworld’s Thandie Newton as DCI Roz Huntley, an ambitious senior detective who, under pressure from her superiors to catch a wanted serial killer, has possibly bent the evidence and arrested the wrong man. “She’s great Thandie... she’s an old pro and full of beans”, says Compston. “It’s a testament to the show that we can attract that calibre of talent.”

However, in a classic Mercurio cliffhanger at the end of episode one, the writer left viewers in suspense as to whether Newton’s character had been killed or not by Jason Watkins’s whistleblowing forensics officer.

Episode two further teased the possibility but [spoiler alert] it suddenly became apparent that the boot was on the other foot. “Cliffhangers… that’s what he specialises in Jed”, says Compston. “Every year I read the scripts and think he can’t do it again, and he has. There are a lot of classic Line of Duty twists and turns to come in this series.”

Compston has made his own twists and turns on the road from Greenock on the River Clyde. His father is a pipe-welder in the shipyards and his mother works for the council. Compston himself, although enjoying acting as a child, never considered it as a potential career until, on a double-date to the cinema with a friend, they missed their intended film and instead caught My Name Is Joe, Ken Loach’s 1998 Glasgow-set drama starring Peter Mullan (now a friend of Compston’s in LA).

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“It just blew my mind seeing these characters that I could instantly recognise”, says Compston. “And they spoke like me, but there on this massive screen. And then Ken Loach was holding open auditions at my school, and at the time I didn’t know who he was, but when I heard it was the My Name Is Joe guy I went ‘wow!’ and gave it a go.”

The film was Sweet Sixteen, in which Compston was perfect as the main character, troubled teenager Liam, and found his face dominating posters for a movie that went on to win the best screenplay award at the 2002 Cannes Film Festival. It was a wonderful start and led to auditions in London for TV roles in The Royal and Monarch in the Glen, and an abandonment of Compston’s burgeoning professional football career.

“The club I was playing for, Morton, which was my local club – a fantastic club and they’re doing really well at the minute – but they were struggling then, and it wasn’t a great atmosphere for a young player to be in”, he says. “You have to be aware of your limitations and I knew I would never have got to be where I would have wanted to be in football. My dream is to play for Celtic and I wasn’t going to do that so I felt I had a better chance of being where I want to be in acting.”

It was a good decision, and now at the age when his football career would have been petering out, if it already hadn’t been curtailed by injury or the usual disappointment of young players not fulfilling their promise, he finds himself filming in Prague with Keira Knightly and Alexander Skarsgard. The Aftermath, based on Rhidian Brook’s international bestseller, is set in the bombed-out Germany of 1946, when a British colonel charged with rebuilding Hamburg is joined in the city by his wife, and end up sharing their apartment with its previous German occupants, a widower and his troubled daughter.

“It’s a fascinating time… so undocumented,” says Compston, who plays the head of British intelligence in Hamburg. “A lot of British soldiers had to stay on and their families came over to Germany to join them.”

Hard at work since his wedding, Compston is off for a spot of kayaking in Utah before returning home to LA, and the wife, model and nightclub manager Tianna, who he met in a bar. “She was working there”, he says. “She’s a mixed race girl with a big mass of hair and is quite striking when you see her. She was wearing a Claddagh ring – an Irish promise ring – and turns out her dad’s Irish and she’s got Irish family so I just sang her a couple of songs from the old country and then we were there.”

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They had a big wedding at a golf spa overlooking the Clyde, between Greenock and Glasgow, with bagpipers and Compston wearing a kilt, and his Scottish identity is obviously important to the actor. He believes that Nicola Sturgeon was right to demand a second referendum on Scottish independence.

“It’s becoming a necessity”, he says. “Scotland’s been backed into a corner. We’ve now been forced into this ridiculous situation where we’ve been forced to leave the EU against our will. I don’t blame people for voting to leave because there’s obviously a section of the country that feels ignored by the political class, but people didn’t know what they were voting for.

“When you’ve got lying bastards putting ‘£350 million extra for the NHS’ on this big stupid bus, and the shameful thing is that it’s going to be the poorest who suffer again.”

It might even surprise some people that Compston is Scottish at all, given his convincing estuarine accent in four series of Line of Duty – although his chilling turn as 1950s Scottish serial killer Peter Manuel in ITV’s In Plain Sight was a form of accent home-coming. “Yeah, more people have found out that I’m Scottish after In Plain Sight”, he says. “I bet this year on Line of Duty I get a lot of grief, because they’ll now sit and listen for mistakes. It’s one reason I like to keep a low profile.”

He does? “Yeah, I do. I don’t mind promoting my work, but other than that I don’t see why you should be in the papers. There’s nothing worse than actors being asked ‘what’s your favourite Christmas?’ or ‘what’s your perfect date night?’ or any of that bollocks. It’s not for me.” Luckily none of those was on my list.

‘Line of Duty’ continues at 9pm on BBC1 on Sundays