He’s credited as “Unseen Arkham Prisoner” in the cast listing, but it’s clear that he’s playing The Joker. In the scene, Barry begins talking with Riddler after he’s thrown in Arkham.

Ledger would famously also keep a diary while becoming the Joker, which detailed his descent into the mind of Gotham's creepiest villain. The diary would later be released by the star's father, Kim ...