This quickly let us know that the character was a scheming, machine-gun-toting mastermind, which completely set him apart from all the other Jokers that we knew, especially Cesar Romero in the silly '60s Batman TV show. That is, other than the fact that Ledger wore Romero-Joker's mask throughout the entire bank robbery.
20th Century Fox Television
All evidence seems to point to the design of Ledger's clown mask being modeled directly on a scene from "The Joker Is Wild," an episode of the Batman TV series that also marked the first appearance of the Joker on the show. In the episode, Adam West's Batman tracks Romero's clown prince of crime to the Gotham opera company, where he's secretly performing an aria from Pagliacci, because back then that was as close as the Joker could get to being an actual killer clown (look it up). As you can see, the similarities between Romero's Pagliacci costume and Ledger's mask are undeniable.
20th Century Fox Television, Warner Bros.