EXCLUSIVE: Zac Efron Goes NSFW Dishing on Robert De Niro in 'Dirty Grandpa'

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Dirty Grandpa
may be one of Robert De Niro's most raunchy, insane roles to date, and even playing a perverted old man, the iconic actor had everyone else on set star struck.

In ET's exclusive behind-the-scenes look at the NSFW comedy, De Niro's co-star, Zac Efron, can't contain his admiration for the legendary star.

"The coolest character in the movie, by far, is Dick, played by the one and only Robert De Niro," Efron shared.

WATCH: Zac Efron Is Now Nearly Naked on the Set of 'Dirty Grandpa'

Aside from being effortlessly cool, De Niro once again got the opportunity to show off his comedy chops, and absolutely killed. According to Efron, it was hard not to crack up during every scene.

"I would break constantly, because the stuff that would come out of his mouth was insane," Efron, 28, recounted.

Even veteran actor Danny Glover was blown away by his chance to share the screen with the 72-year-old Oscar winner, who said his scenes with De Niro are definitely "gonna be in my sizzle reel."

WATCH: Zac Efron Talks About Being Intimidated Working With 'Legend' Robert De Niro

For some unsavory details about one scene involving Efron's butt and De Niro's thumb, watch the clip now!

Dirty Grandpa
hits Blu-ray and DVD on May 17, and is available on digital HD on Tuesday.

WATCH: Zac Efron and Robert De Niro Go Shirtless, Flex Their Muscles in 'Dirty Grandpa'