Watch: Martin Compston pranks Vicky McClure on Line of Duty set
Line of Duty fans have finally discovered something more confusing and complex than the search for criminal mastermind H — Martin Compston's real accent.
The 36-year-old star performs a London dialect as DI Steve Arnott in the BBC crime show, but actually hails from Greenock in Scotland.
Appearing alongside co-star Vicky McClure on Saturday's episode of The Jonathan Ross Show, Compston caused a stir when viewers heard his normal accent, which is vastly different to his character's.
"The most surprising thing on the Jonathan Ross Show tonight was Martin Compston's Scottish accent," wrote one viewer on Twitter.
Another fan wrote that their "brain does not compute" hearing Compston's natural accent after almost a decade of hearing him portray Arnott.
Meanwhile, one fan posed that the star's accent could even play into the show's mystery, with Arnott dropping his false English accent to be shown as an ally of Scottish criminal gang member Tommy Hunter.
They wrote: "This is far-fetched but hear me out. Arnott is H but here’s the twist: at the end you stop putting on your English accent and revert back to your true Scottish accent to somehow be in ties with Tommy Hunter."
Compston revealed on the show that it takes all of his acting chops to maintain Arnott's accent while delivering creator Jed Mercurio's trademark, knotty dialogue.
"That dialogue on its own would be hard anyway, throw the accent in there," he said.
Compston added: "It's just graft. I'm not naturally good at accents. I need to work harder, which then makes me better because I'm not complacent with it.
"If you're in these scenes with these amazing actors, amazing guest stars, it's intimidating enough without worrying about the accent. Speaking to the wife and stuff I'll drop it, I very much stay in it [on set]."
The final episode of Line of Duty's sixth series is due to air on BBC One tonight, with those behind the show teasing that fans will finally get an answer to the identity of criminal kingpin H.
Fans are inevitably delivering theories aplenty about the show's various mysteries, including suggesting we may not have seen the last of James Nesbitt's short-lived character Marcus Thurwell.
Read more: Line of Duty fans spot H spelling clue
Last week's penultimate instalment was watched by 11 million people — more than watched the Oscars in the USA.
The episode featured an extended interview with suspected copper DCI Jo Davidson (Kelly Macdonald), in which she answered "no comment" more than 30 times to avoid incriminating herself.
There have been suggestions that this could be the show's final series, with Compston telling the Shrine of Duty podcast that even he isn't sure whether there will be more from AC-12.
He said the decision rests with Mercurio, who "always takes a couple of months" after a series airs to determine whether the show should continue.
Compston said: “There is a lot of stuff that’s way beyond my pay grade — the data, the figures, the analytics, audience scores, all that.
"I think [Jed] likes to take the emotion out of it and sit down and look at all that."
He added: "Also when we get 10 years in you start to think of legacy, and if it goes down that well in some ways it might be the perfect ending."
Watch: James Nesbitt schemed with Jed Mercurio to hide Line of Duty appearance