2003's The Hulk (starring Eric Bana in the title role) and 2008's The Incredible Hulk (starring Edward Norton)
You won't like him when he's angry. Bruce Banner, an unassuming, genius scientist, becomes exposed to gamma radiation, which transforms him into a giant, smashing monster when he gets upset. Banner has battled his father (Nick Nolte), his girlfriend's father (first Sam Elliottt, then Hurt), and his girlfriend's father's soldier-turned-mutated-creature (Tim Roth). All this activity has gained the attention of Fury and Stark.
The Hulk has long been rumored to be the villain of the movie, the one that helps bring the team together (much like the comic book origin of the team). While that may yet hold up as true, all previous speculation went out the window once Joss Whedon was named director and screenwriter — Zak Penn was the movie's first screenwriter, but what will be used from his draft is unknown and Penn was not mentioned in Marvel's start of production announcement. When Ruffalo was asked last December whether the Hulk was the villain, the actor responded, "He may or may not be."
One thing's for sure: While Ruffalo may be the third actor to play Banner, he will be the first to also tackle the motion-capture process used to create the CG Hulk. Ruffalo has described the process as standing in front of a monitor, seeing himself as the Hulk, and raising a "a 250-pound right arm, with all that weight and mass." The Hulk will even look like Ruffalo, the actor revealed to Advocate recently, even down to Ruffalo's "scars and wrinkles." The Hulk may even have hair on his chest. "No one's asked me to shave my chest yet," said Ruffalo. While talking to Entertainment Weekly, Ruffalo said the Hulk "looks just like me, but bloated and green." And with a hairy chest, apparently.
Ruffalo has said that the Hulk will have unusual relationships with the other Avengers, specifically citing an "interesting" dynamic between Banner and Tony Stark that has Stark enjoying "the idea of watching this guy turn into The Hulk." Ruffalo also said the Hulk has "an interesting thing" with Black Widow (Scarlett Johansson), and later described the Hulk as "the teammate none of them are sure they want." The latter comment implies that the Hulk, at some point, will be integrated into either becoming a member of the team or at least working with them.
Ruffalo has described his preparation for the role as losing 15 pounds and watching "100 hours" of the incredible Hulk TV show starring Bill Bixby.