For "The Dark Knight Trilogy," director Christopher Nolan wanted to make a "logical" Batman story. That meant not only crafting an internal psychology for Bruce Wayne but also dragging the mechanics and aesthetics of his world down to reality. There's a reason the films spend so much time explaining how Batman's gear works (Lucius Fox (Morgan Freeman) is turned into the Q to Batman's James Bond) and turn Gotham City from a Gothic metropolis into basically just Chicago with a different name.

This mandate for realism shaped the other characters in Batman's world, especially his villains. In "The Dark Knight," the Joker's (Heath Ledger) clown visage — white skin, green hair, ruby lips — is only makeup. His disfigurement is a Glasgow smile; Ledger's Joker frowns a lot more than in the comics, but those scars mean he never stops smiling.

Ledger's performance is now legendary for how he immersed himself in the role; before shooting,...