In the Comics: Physicist Bruce Banner survives an explosion of gamma radiation in his lab, becomes an invincible green giant when he gets angry.
But in the Movie: Bruce's (Eric Bana) crazy father David (Nick Nolte), also a scientist, conducts human trials on himself, mutating his DNA and passing it on to his son. Later, adult Bruce's exposure to gamma radiation completes his transformation into the green beast.
Does the Change Work?: Yes. Ang Lee's introspective take on the Hulk isn't a slam dunk, but his intentions were strong. Making David Banner (who was established as abusive in the comics) a bigger factor in Bruce's origin gives the story more emotional complexity and trauma than you'd expect from a movie about a guy who turns green and smashes things. No, really.