The actors bonded during filming for Avengers: Age of Ultron

By Sheila Cosgrove Baylis
Updated April 27, 2015 10:35 AM
Credit: ChinaFotoPress/Getty

Mark Ruffalo and Robert Downey Jr. are bros, on-screen and off.

The pair star together in Joss Whedon’s Avengers: Age of Ultron and got a bit crazy together during filming.

“He’s been just a bro to me,” Ruffalo – who plays Bruce Banner, a.k.a. The Hulk – tells PEOPLE. “He also, he thinks I’m a little insane. So when we get into those moments, we are half laughing through the whole thing. He’s half like, ‘Ruffalo, how do you even get up out of bed?’ ”

“He has the most endearing sort of patience and tolerance for me,” the actor continues. “And we literally are laughing behind everything. I don’t know what’s going to come up with him, which makes it so much fun and alive. We also know that we’re playing on this theme of the science bros that’s so wildly loved. And so we’re playing that up. You sort of understand that whole phenomenon. He’s like a brother to me.”

The actor, 47, adds that Downey Jr., 50, who plays Iron Man, supported him through his fears of joining the franchise.

“He’s kind of my godfather in the world. He sort of gave me the okay to come join and has sort of bolstered me along the way when I’ve had my doubts, which are plenty. And he has made sure that Banner’s been well represented.”

Reporting by SCOTT HUVER

For more on Mark Ruffalo and Robert Downey Jr.’s friendship, check out the current issue of PEOPLE, on newsstands now