Line of Duty actress Vicky McClure pranked Scots co-star Martin Compston by drawing a penis inside one of his props used for the show.
McClure, who plays DI Kate Fleming on the hit cop drama, drew the rude image inside one of the Greenock-born actor’s police notepads.
And he only managed to clock it in the middle of filming a scene.
Compston will be returning to our screens as AC-12’s DS Steve Arnott when the sixth series of the BBC programme airs later this month.
Speaking ahead of the new series, McClure told That Gaby Roslin Podcast: “I remember one year, Martin got his little police notebooks and I’d drawn (the image).
”It's stupid, so childish and it's not funny for anyone else!"
Compston added: "It happened again this year!"
"They just gave me the same notebook. It's just a random page and I just opened it...
"I always start these scenes (by saying) 'AC-12 interview"' Compston said, referring to the name of the anti-corruption unit, "and I'm just flicking through (and see it).
"You're trying to cover it and hide it and you don't want to tell anybody else, because everyone will go (laugh), so you just have to hide it," he said.
Compston said there is much corpsing on set, especially in the "huge interview scenes", which are so "daunting and stressful".
"There's always a point... somebody says something and then we're gone," he said.
"It takes a good 20 minutes and we just laugh it out and then get back to it."
He said the "righteous rants" of Ted Hastings, played by Adrian Dunbar, can set them off.
Compston said that Dunbar "shouted: 'There's a herd of elephants coming for you.' It's not in the script.
"It came out when he was on a rant. We all tried to keep it together and then somebody went and we were all gone. It's a release. What's funny is people trying to hold it together. "
Line of Duty returns to BBC One on March 21.
"In some way the whole series has been building to this," Compston said of the next instalment.
"I think there are a lot of big answers coming in this series".
The Line of Duty episode of That Gaby Roslin Podcast is released on March 8.
