HE’S become the leading man in one of the most popular television dramas of the last decade, yet he’s still hunting for the essence of the 17-year-old boy he was.

Actor Martin Compston appeared this year alongside Margot Robbie and David Tennant in Mary Queen Of Scots, one of the biggest Scottish films of recent years.

He has picked up a slew of acting awards since his debut in Ken Loach’s Sweet Sixteen as a raw Greenock teenager.

But none of that counts when the latest director rolls the cameras.

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The 34-year-old, who returns to screens tomorrow night in the fifth series of police drama Line Of Duty, admits he still feels “the fear” every time he steps foot in front of camera.

He said: “I suppose it’s good thing as an actor. You’re never satisfied. That’s why I’ve never watched a film after a premiere. I sit there thinking about what I should have done differently.

"I’ve not seen Sweet Sixteen for about 15 years. I’ll watch something once then not again, because I’ll just pick it apart.”

Mr Compston was pursuing a nascent football career on Greenock Morton’s youth books when he was plucked from obscurity by veteran director Ken Loach’s casting producers to play Liam, the doomstruck teen in the warts-and-all tale of family, love, poverty and addiction.

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The job thrust him into a spotlight he hasn’t strayed from since, with roles alongside Robert Downey Jnr in US indie flick A Guide To Recognising Your Saints and James McAvoy in the movie version of Irvine Welsh novel Filth.

He said: “I’m actually still trying to get back to the kid I was at 17, when I was fearless. It’s a bit Catch 22. You have to know about angles and lenses and how to hit your mark, and what to do when the camera is on you.

“But when you’re a teenager doing Loach’s stuff, you didn’t care about that, you just acted and didn’t care who the camera was on.

“I can’t believe what he did for me. He let me run riot, knowing what I know now. I was calling shots, I was changing things in the scripts, and I didn’t know any better. But he said ‘it’s your gut, go for it’. I suppose in some ways that’s why he cast me.”

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Despite since working with some of the best in the business, and playing DS Steve Arnott in Jed Mercurio’s drama, the actor is mindful of complacency.

“You have to guard against that, especially after the success of something like Line Of Duty,” he said.

“They call me ‘No 1’ over there. I’m first on the cast list and I take that very seriously. In some ways, you have to drive the show through, like you’re the captain of the thing.

“If we do this right, we’re on the verge of becoming one of the greatest TV shows of all time, if we eventually finish it the way I know Jed can. It will be something I can look back on in my 60s and 70s and think ‘I’m still incredibly proud of that’.”

The actor also revealed he has turned down approaches to appear on reality television shows.

He said: “They come in and offer money but I’ve no interest. I’m privileged to be in that position, but I have no interest. It’s not what I do. I take my job very, very seriously.”

Compston lives in Las Vegas, Nevada, with his American-Irish wife Tianna Chanel-Flynn.

The pair relocated from Los Angeles last year, with Compston spending several months a year filming Line Of Duty in Belfast.

One role on the horizon might demand that he spends a bit more time in Vegas himself.

He said: “We’re thinking about starting a family soon.

“We were going to come home to Scotland, but I was very aware of having seen some pals, and what they and their wives were going through, without that support network.

“So it needs to be somewhere where there’s support when I’m gone.”

Line Of Duty returns to BBC1 tomorrow.