Mark Ruffalo was initially hesitant to play Hulk, but some encouragement from Robert Downey Jr. was all it took for him to say yes.

The 52-year-old actor initially did not see himself as part of the Marvel Cinematic Universe, especially one who played the character of Hulk. He thought he could not match up to the other actors who already did great in their respective versions of the angry green giant. Hence, he was hesitant when offered the role in "Avengers Assemble."

Ruffalo admitted in a video interview for "The Tonight Show starring Jimmy Fallon" that he "was scared." He said he did not know what else he could offer if he was to play Hulk.

"I didn't know what I could add to what I already thought had been done so well before me. So I was like, 'I don't know if I'm the right person for this,'" Ruffalo said, adding that "up 'til that point" he had "only been doing indie movies."

However, "Avengers Assemble" director Joss Whedon assured him that he is perfect for the role. He said Ruffalo is the "right person" to play Hulk. The actor remembered that Downey phoned him afterwards to further convince him to take the part.

"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 shared.

Ruffalo also revealed that before taking on the role he made sure to read the script first. He was refused several times when he asked for it. Thankfully, Whedon provided him with 20 pages of what he wrote for Bruce Banner. He immediately fell in love when he read the first scene when Black Widow (Scarlett Johansson) tracked down Hulk in India.

Prior to appearing in "The Avengers," Ruffalo and Downey Jr. both worked together in the David Fincher film "Zodiac." Aside from Ruffalo, other actors who portrayed Hulk included Edward Norton and Eric Bana.

Mark Ruffalo in Avengers Assemble
Mark Ruffalo in "Avengers Assemble" Film Frame / Marvel