Line Of Duty is as well known for its twists and turns as for its famed catchphrase 'bent coppers', and last night's episode certainly delivered on that score, with actor James Nesbitt popping up in the form of dodgy retired detective - and new H suspect - Marcus Thurwell.
And to ensure that his role in Line of Duty remained a surprise, Nesbitt earlier carried out a bit of acting off-screen by telling The Radio Times he was p***ed off that he'd never been invited to star in the hit show.
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Speaking while promoting his show Bloodlands, which was filmed in Belfast, Nesbitt said: "I said, 'Come on, Jed [Mercurio, LoD writer], when are we going to get a chance to work together?'
"Even though Line Of Duty had been shot here for years I was never considered for it, which has always slightly galled me."
Sneaky, eh?
Mercurio, Nesbitt and other members of cast and crew managed to keep the whole thing under wraps until he appeared on screen last night - but they came close to being rumbled when members of the public spotted Nesbitt during filming.
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However, quick-thinking Mercurio simply told them he was doing a bit of modelling for a catalogue.
Mercurio told The Sun: "It was easier to keep it secret by plotting the scheme between the two of us rather than involving a proper casting process."
He added: "I think people thought Jimmy was shooting an advert. I told passers-by he was modelling for a catalogue.
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"The rest of the filming with Jimmy was done away from the public eye so we were able to keep his involvement totally top secret."
Talking to The Sun, Nesbitt said: "I had a lot of fun saying how Jed has snubbed me for Line Of Duty, all the while knowing our little secret."
Mercurio said: "It worked brilliantly, because everyone thought he was genuinely annoyed.
We laughed lots about all the coverage he got for being 'snubbed'.
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But if you're hoping for some more plot details from Nesbitt now the cat's out of the bag, then I have bad news, as the actor added: "I've kept it to myself for over a year and couldn't even tell my closest friends or family.
"I'm still sworn to secrecy about Marcus Thurwell, but as ever with Line Of Duty, there will be a few surprises along the way."
Featured Image Credit: BBCTopics: TV and Film, Celebrity, Line of Duty