The Cold Feet star was a shock addition to the cast in episode five of the police drama's sixth series, when DI Steve Arnott (Martin Compston) looked up detective Marcus Thurwell's police record, showing Nesbitt pictured as the character.
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As well as pretending to be angry about missing out on a role in the hit series, Nesbitt also lied to fans that he was taking part in a catalogue shoot while filming scenes of his character supposedly in Spain.
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The Sun reported that Nesbitt said: "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.”
The hit show's creator Jed Mercurio said he and Nesbitt had kept the plot development between the two of them and that he warned his new star that his role would involve two conditions – wearing shorts, and "he’d have to lie about it".
Northern Irish actor Nesbitt recently starred in Bloodlands, which was produced by Mercurio, and said that both shows filming in Belfast had provided the perfect cover for him.
He threw reporters off the scent during filming telling Radio Times in February this year: "Even though Line Of Duty had been shot here for years I was never considered for it, which has always slightly galled me."
As dodgy detective Marcus Thurwell was introduced to the police procedural, surveillance photos of the character apparently enjoying his retirement in Spain while wearing shorts and a Hawaiian shirt were shown.
— Daniel Mays 💙 (@DanielMays9) April 18, 2021
The scenes were actually filmed at Belfast Marina, and Mercurio admitted: "I think people thought Jimmy was shooting an advert. I told passers-by he was modelling for a catalogue."
Nesbitt's character features in a new part of the storyline connected to an old case about the murder of a character called Lawrence Christopher, which his team worked on and which was investigated for police systemic racism.
Thurwell and the officers he worked with – who included Ian Buckles and Philip Osborne – also appear to be connected to the OCG, who are blackmailing members of the police over their dodgy dealings.
Watch: Martin Compston shares Line Of Duty pranks with Vicky McClure