Dr. Bruce Banner from Marvel Comics lives in constant fear of turning into the enormous green rage monster Hulk, and as Mark Ruffalo revealed on a recent episode of The Tonight Show with Jimmy Fallon, he was equally afraid of taking on the role of Banner/Hulk, because of the legacy attached to the character and the larger MCU.

"I was scared. I didn't know what I could add to what I already thought had been done so well before me. I'd only been doing indie movies up 'til that point. So I was like, 'I don't know if I'm the right person for this.' And Joss Whedon's like, 'Yeah, you are the right person.' 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.'"
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Before becoming an Avenger, Mark Ruffalo was known for a string of well-received roles in small budget movies. Going from that to being front and center of the most successful movie franchise in history was something the actor had to think long and hard about. But Robert Downey Jr., who plays Iron Man, one of Banner's closest allies in the MCU, helped put to rest his fears about taking over the role from Edward Norton.

But Ruffalo still had one other prerequisite before taking on the role: reading the script for his first MCU movie, The Avengers. This requirement was complicated by Marvel Studios' famous reticence when it comes to letting outsiders know anything about their plans for a film.

"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 so I was like, 'I need to read something,' and the studio was like, 'We're sorry, we don't give anything out.' And Joss, was like, 'Here man, here's like 20 pages of what I have written of Banner,' and I read that first scene of when Scarlett [Johansson], when Black Widow finds Banner in India, and I was like, 'Okay, I love this. This is amazing.'"

The scene in question takes place in a dilapidated shack in India, where Banner has been hiding out, helping people while keeping his monstrous alter ego in check. Black Widow turns up asking for Banner's aid in stopping Loki. In that one scene, through a combination of excellent writing and mastful acting by Ruffalo and Scarlett Johansson, audiences are able to get a grasp on the wariness exhibited by Bruce Banner, his reluctance to come out of the shadows and join the Avengers, and Black Widow's mix of caution and fear about dealing with this meek-looking man who happens to be the most powerful being on the planet.

For a lot of the fans, that was the scene that put the seal of approval on Ruffalo's take on the character, and the actor later paid forward Downey's favor in convincing him to become Hulk with Josh Brolin. When Brolin was offered the role of Thanos, he called his friend Ruffalo, who assured him that joining the MCU was well worth the effort, and so Brolin went on to present audiences with a masterful portrayal of the MCU's greatest villain Thanos. People.