(Robert Downey Jr.)
2008's Iron Man and 2010's Iron Man 2; cameo in 2008's The Incredible Hulk
Tony Stark is a billionaire industrialist who is captured by Afghan rebels and creates a suit of armor in order to facilitate an escape. Once back home, Stark continues to develop the suit, fights off a company rival (Jeff Bridges), and eventually admits, publicly, that he is Iron Man. Stark refuses to hand over the Iron Man armor to the government and he ends up facing another rival (Mickey Rourke) in an armored suit. With the aid of his friend Lt. James "Rhodey" Rhodes, aka War Machine (Don Cheadle), he defeats the latest nemesis. Stark is told by Nick Fury (Samuel L. Jackson) that his Iron Man armor would be a benefit to the Avengers Initiative, but he would not.
Director Jon Favreau has previously said that his two Iron Man movies take place chronologically before The Incredible Hulk, so Stark's cameo at the end of that movie — where Stark talks to General Ross (William Hurt) about "putting together a team" — means that Fury has somehow changed his mind and Stark is now in the Avengers.
As the only character in the movie besides the Hulk to have two movies already, expect Downey Jr. to play a prominent role in the movie — he's the hero audiences are most familiar with. Downey Jr. was already instrumental in convincing his Zodiac co-star Mark Ruffalo to take the role of Bruce Banner in the movie.
Basically, nothing. Besides joking last November that Whedon's script needs work, Downey Jr. told Access Hollywood before the Golden Globes that "we're going to do something extraordinary." Then in March, he told MTV that preparing to play Tony Stark was easy, only requiring him to "grow a mustache."