To transform, hold
as the following characters:The Incredible Hulk
: Unlocked from the beginning. If you keep holding
, you'll transform between Bruce Banner and Hulk. Do it 50 times for another achievement.Red Hulk
: Not sure exactly when he's available, but I found him post game near the X-Mansion while still in the city. He's on the same coast as Roxxon and the Thor levels.Venom
: Unlocked after level 3, Exploratory Laboratory.Curt Connors
: Unlocked from the extra level Reptilian Ruckus inside Central Park, opened at 125 gold bricks.Stan Lee
: Rescue all the Stan Lees in Peril. Transforming as him earns another achievement.