While speaking to Fallon, Ruffalo explained that he was nervous to play Bruce Banner after Edward Norton had already portrayed the character in Ang Lee's 2008 film The Incredible Hulk. "I was scared," Ruffalo admitted. "I didn't know what I could add to what I already thought had been done so well before me."
Ruffalo also noted that up until playing Bruce Banner in The Avengers (2012), he had mostly been working on indie films and didn't know how he felt about being in such a big blockbuster. "So I was like, 'I don't know if I'm the right person for this.' And [The Avengers director] Joss Whedon was like, 'Yeah, you are the right person,'" Ruffalo recalled.
However, the three-time Academy Award nominee credits co-star Robert Downey Jr. with ultimately being the person who persuaded him to join the cast. "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've got this.' In true Iron Man fashion," Ruffalo said. "And then after that, I was like, 'I guess I have to do it.'"
Ruffalo also talked about how Marvel's secrecy surrounding their projects made him question if he wanted to come on board as Bruce Banner. "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. "And so I was like, 'I need to read something,' and the studio was like, 'We're sorry, we don't give anything out.'"
At the end of the day, Whedon came to the rescue by giving the actor 20 pages of what he had written so far for Bruce Banner's character. "I read that first scene ... when Black Widow finds Banner in India, and I was like, 'Ok, I love this.' It was different, you know," Ruffalo said of the script. "I was like, 'Oh, ok ... It's humorous and it's kind of playful and I think I could do something with it.'"
It's hard to imagine anyone else playing the Hulk now that Ruffalo has put his stamp on the part, and even the actor seems reluctant to step away from the role. Ruffalo has already talked about the possibility of returning to the MCU as Banner in the upcoming Disney+ series She-Hulk.