Vicky McClure has revealed the prank she played on Line Of Duty co-star Martin Compston – drawing a rude image in his police notebook.

The sixth instalment of the nail-biting BBC drama returns at the end of March.

McClure said that she drew a penis inside Compston’s prop.

Her co-star, who plays DS Steve Arnott, only saw it when he filmed a scene, 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!” the actress, who plays DI Kate Fleming, said.

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.