"I'd always felt that Mark was the guy to play Bruce Banner," Downey Jr said, reports Flicks and Bits.
He continued: "I think because there's something immensely introspective and affable about the guy, that makes that 'curse' or that 'opportunity' or that 'karma' that he has, which is not of his doing, but is his responsibility to manage in some way.
"He's just a really funny, quirky guy – but he's a straight shooter. That's probably the most important thing , you know? He was great to work with."
Downey went on offer an explanation as to why his character Tony Stark would join with other super-heroes to battle evil.
"I think he's curious. It's hard for him to ingest what's going on, so he wants to see it first hand. Although he's still himself, with the storytelling side of things, he's searching," he revealed. "In Iron Man 2,'with Rhody, he realised he's not an island, and so there was a partnership, and the same can be said with Pepper."