Actor Mark Ruffalo says Iron Man star Robert Downey Jr. convinced him to take on the role of the Hulk aka Bruce Banner and join the Marvel Cinematic Universe (MCU) in The Avengers.

The actor, who has been an integral part of the MCU since 2012, made the revelation during an online appearance on Jimmy Fallon's chat show.

"I was scared, Ruffalo 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. 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," he added.

Ruffalo said he was approached by The Avengers director Joss Whedon, who told him he'd be perfect for the part, and then Downey Jr called him to do some cajoling.

"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," he said.