James Nesbitt has admitted lying about not being approached to appear in Line of Duty by show creator Jed Mercurio.
The actor made a surprise appearance in last night's episode of the cop drama as Marcus Thurwell, a retired senior investigating officer with links to the OCG and the cover-up of a boy's death while in police custody.
Nesbitt plays the lead in Bloodlands, another police drama produced by Mercurio, and while promoting that series earlier this year he claimed to be offended that he had never been cast in Line of Duty.
"I had a lot of fun saying how Jed has snubbed me for Line of Duty, all the while knowing our little secret," the actor told The Sun.
"I've been a fan of Jed's work for years which is how my role in Bloodlands came about. When he approached me for Line of Duty, of course I immediately said yes.
"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."
Mercurio added: "It was easier to keep it secret by plotting the scheme between the two of us rather than involving a proper casting process.
"If anyone spotted Jimmy arriving on set or leaving, we just said he was shooting extra scenes for Bloodlands."
Line of Duty continues Sunday at 9pm on BBC One.