Mark Ruffalo joined Jimmy Fallon for a virtual chat on Friday’s “Tonight Show”, and revealed how Robert Downey Jr. convinced him to take on the role of the Hulk and join the Marvel Cinematic Universe in “The Avengers”.

“I was scared,” he said of stepping into the role, which had previously been played by Edward Norton in 2008’s “The Incredible Hulk”, Eric Bana in 2003’s “Hulk” and Lou Ferrigno in the 1970s TV series.

“I didn’t know what I could add to what I already thought had been done so well before me,” Ruffalo told Fallon. “And I had only been doing indie movies up to that point so I was like I don’t know if I am the right person for this role.”

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Ruffalo was approached by Joss Whedon, director of “The Avengers”, who told him he’d be perfect for the part. “Joss Whedon’s like, yeah, you are the right person,” he recalled.

“And then I got a call from Downey, it must’ve made it to him that I was hemming and hawing, and he just simply said, ‘Ruffalo, let’s go. We got this.’ In true Iron Man fashion. And then after that, I was like, ‘I guess I have to do it,’” he said.

Despite Downey’s assurance, Ruffalo admitted he was still hesitant. “I wanted to read a script, and they don’t give you scripts. I made a promise when I was a young actor that I would never do another movie without reading a script beforehand, because it’s a recipe for disaster, and then you get stuck in something — our only power as young actors is our ability to say ‘no’ sometimes,” he explained.

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“I was like, ‘I need to read something,’ and the studio was like, ‘We’re sorry, we don’t give anything out,” he said. Finally, Whedon snuck him about 20 pages of some scenes he’d written featuring the Hulk and alter-ego Dr. Bruce Banner.

“I read that first scene of when Scarlett [Johansson], when Black Widow finds Banner in India, and I was like, ‘Okay, I love this,” he said. “It was different, you know. I was like, ‘Oh, okay… It’s humorous and it’s kind of playful and I think I could do something with it.'”

While the Avengers are no more, Ruffalo says he’s been pitching Marvel on a standalone Hulk movie that would showcase what Hulk/Banner was up to in between “Avengers” movies, and delve deeper into how Banner and Hulk merged into what Ruffalo calls “the Bulk.”

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“We’ve never really followed him into his life. He’s always kind of off on the side. He’s like the Rosencrantz and Guildenstern of the Avengers,” said Ruffalo, comparing his character to the characters in Shakespeare’s “Hamlet”. “It’d be interesting to fill in all the blanks about what happened to him in between all these movies.”

He added: “That’s what I’m pitching actually, that section of time, like how did Banner become the Bulk? Half-human, half-Hulk.”

Later in the interview, Ruffalo discusses playing twin brothers in new HBO series “I Know This Much Is True”, and explains why he’s working with Protect the Sacred to support Navajo Nation elders and others most vulnerable to COVID-19.