Zac Efron seems genuinely dazzled by the fact that he's now made a film in which he appears alongside with one of cinema's finest actors. "Robert De Niro is the cool dude of all cool dudes," Efron enthuses about his Dirty Grandpa costar. "He's never seen a single one of my films, guaranteed!"

Efron plays the grandson of De Niro's characterEfron plays the grandson of De Niro's character

He says that he was so nervous about meeting him that it felt like going on a first date. "Do I come on really strong or do I play it cool, as if it's every day that you meet someone like Robert De Niro?" he laughs. "We had to find out each other's vibe and the way we worked. And then halfway through the movie we sort of just clicked. Basically if you talk to him about anything other than acting, he's an open book. He loves talking."

Efron calls Dirty Grandpa "a twist on an old formula, a crazy adventure about a guy who has life figured out and things are looking good for him, and then his grandpa asks him for a favour." Where this goes is unexpected, as De Niro's Grandpa shatters expectations with his raucous behaviour, all while teaching his grandson how to live.

"It was fun to see the one and only Robert De Niro inhabit this character," Efron says. "I'm a pretty big fan of his. He's been doing this for so long and is such an icon, and yet I don't think I've ever seen him do a role like this. He's totally committed to letting go in this movie, and it was really exciting to me to just observe that and play into it. I got to essentially be his straight man! He's formidable, dramatic and real, and when he applies those talents to comedy, the screen just blows up."

Since much of the film takes place in Daytona Beach on spring break, both men spend rather a lot of time with their shirts off. And the ripped-as-usual Efron was shocked that De Niro is in such amazing shape. "This is another reason he's a legend," Efron says. "He wakes up at like 4 in the morning every day and works out with a trainer, then has coffee, then goes to work at like 6. I don't know how he does it. I promise, De Niro could kick my ass!"

Watch the trailer for Dirty Grandpa here: