In case you wanted a reason to go goof off in Arkham City again, the folks at Rooster Teeth have taken the time to show us how to activate Big-Head mode in Rocksteady's gritty comic book adventure.
It's very easy to do. You have to be playing on normal or hard, or on New Game+ (Which you can't unlock by completing the game on easy, anyway.) At any point in the game, take out Batman's Cryptographic Sequencer. While holding down both RT and LT (or, I presume, R2 and L2 on PS3), begin to rotate your left thumbstick clockwise and your right thumbstick counter-clockwise. After doing that for a few seconds, you'll notice that the Dark Knight's chin has gotten a good deal… chinnier.
And now you're free to run around an Arkham City that is significantly more ridiculous and adorable. To turn off Big-Head mode, simply open the Cryptographic Sequencer and follow the activation steps a second time.