Kieran Culkin admitted he once swapped a prop joint for a real one while on set with Mark Ruffalo

Kieran Culkin has admitted to swapping out a prop joint for a real one during a play with with Mark Ruffalo.

Back in 2012, Avengers star Ruffalo revealed during an episode of The Graham Norton Show that he'd accidentally gotten high during the opening night of their off-broadway production The Moment When when one of his co-stars cheekily switched out his prop joint for a real one.

"On the opening night with all the critics he slipped a real joint onto the prop table," he explained.

Unaware of the switcheroo, Ruffalo proceeded to get very high with his co-star, adding: "Between the two of us we smoked a giant blunt on stage and I'm like 'Man I am in it tonight, I'm on fire man'."

"And the kid who put the joint on the prop table standing there [laughing] and I just went 'oh s**t'. I got the best reviews of my entire career. But I’d never do that again."

For years the identity of this particular young actor remained a complete mystery - well, that's until now.

Kieran Culkin made the admission during a recent interview (Axelle/Bauer-Griffin/FilmMagic)

During a recent interview with The Guardian, Succession star Culkin revealed that he was the one responsible for getting Ruffalo high during the production and seemed a little remorseful about his actions.

"I was 17 and stupid," he admitted. "I’m like, ‘I thought this was a good prank. I’m stupid. Oh my God, I’m so sorry.’ But actually, they loved it."

He then went on to recall how Ruffalo had joked 'The second half’s going to be so much fun' after he learned the truth about what had happened, while another one of their co-stars revealed it was the first time she'd ever been high.

Which is certainly as well as Culkin could've expected a risky prank like to have gone down.

Fortunately, Ruffalo took the risky prank in his stride (Monica Schipper/Getty Images)

Unfortunately the production's stage manager was less than impressed by the stunt.

"The stage manager comes stomping in and goes, 'I don’t care whose it is, or what happened, but Kieran, give me the joint'. I sheepishly handed her the roach and she said, ‘Ruin your life on your own time,'" Culkin recalled.

Thankfully, any of Culkin's current and future co-stars can rest easily as the actor has now said it wasn't the wisest thing for him to do.

"I know, I know. But I was young. I’m 42 now. I know better. I’m not going to try to get anyone high on stage," he added.