The Avengers star told journalists at the movie's UK press conference that his Bruce Banner will "bust out into the big green rage machine".
Discussing Joss Whedon's approach to the Marvel character, Ruffalo commented: "He said he wanted to return to the Bill Bixby Hulk, this world-weary charm of a man who's on the run but at the same time living his life.
"I liked the idea that I would be the first actor to play Bruce Banner and the Hulk [through motion-capture]. That was probably the most exciting thing to me."
Ruffalo quipped that his motion-capture leotard made "all the wrong places look big and all the right places look small".
The Avengers is released in UK cinemas on April 26 under the title Marvel Avengers Assemble and on May 4 in the US.
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