Complete all the single mission dossier objectives. Then during play, hold Back and enter the following:

Invincibility (self):
X, X, A, B, A

Invincibility (team):
B, A, Y, Y, A, B, X, X, X

Big Heads:
A, X, B, Y,A

Chicken Heads:
X, X, Y, A, B

Flat People:
B, A, X, Y, A

Helium Mode:
X, A, Y, B, X

Slow Mo:
Y, Y, B, X, A

All heroes:
Beat first twelve missions in Campaign to unlock all twelve heroes as playable characters in Mission and Campaign modes.

Thanks to Revolution readers Gareth John, Sam Toshner, Marc Bashir, billybob, Dave Beazley , and Abe Viggo!