Martin Compston hints at Line of Duty season 6 twists: "It's very exciting where it's going"

"I think that's the thing with the show... there's so many grey areas," said Compston, who plays Steve Arnott. "That doesn't mean that he [Hastings] is the big bad, but whether he's been up to something shady at some point, that's a possibility.

"I need to wait to read the scripts, but from everything I've seen and what I've been told, it's very exciting where it's going next year."

Another plot left open was the fate of OCG member Ryan (Gregory Piper) – the young crook was last seen being accepted for training as a Student Police Officer, allowing criminal elements to exert a greater influence over the police service.

Ryan and Steve Arnott famously had a run-in during Line of Duty's first series, with the juvenile gangster attempting to chop off Steve's fingers with a bolt-cutter. Compston wouldn't be drawn, though, on whether the two will face off again in future...

"That's the beauty of it – all these little things Jed [Mercurio, the show's writer] leaves in there. I couldn't even tell you if the character Ryan will come back, but if he does or doesn't, then you know he's there – he's planted, and it's just something that keeps ticking over in the background."

Heading into its sixth series, Compston suggested that Line of Duty's "biggest enemy is complacency", saying: "That's across the board, whether it be the scripts, the casting, or just a simple thing like my accent.

"We need to come out harder and better, every time."

The Scottish actor affects an English accent to play Arnott and though many viewers don't realise the truth, Compston insisted that he's determined to keep working on his 'Steve voice' until it's "flawless".

"You worry now that people [will] start listening for it," he said. "That's the thing with online... you can get a million good comments, and then it's that one [negative comment] that drives you up the wall. That's the one that sticks with you.

"That's the problem, because people go, 'Episode four, minute 32, you said this word wrong' and that's really how pedantic people get about these things. So that's just maybe a bit of a worry, but that just makes me double down and work harder."

Martin Compston will appear in Traces, beginning this December on Alibi