Just as Batman is one of the most popular superheroes of all time, his Arch-Enemy the Joker is one of the most popular supervillains. And much like how the Batman Parody is an Expy of Batman, the Joker Impersonator is modeled after the Clown Prince of Crime. A chaotic and unhinged villain with a tendency for slasher smiles and a usually bombastic attitude, the Joker Impersonator tends to have at least a few of the following traits:
They're a Monster Clown, usually accompanied with a macabre sense of humor and a Faux Affably Evil act.
Dress in loud colors, like purple and/or green.
Are completely Ax-Crazy, to a point where other villains are disturbed by them. May or may not be pure evil.
They commit evil acts just because they feel like it.
Believe in a Straw Nihilist philosophy or act as agents of chaos.
Have a disturbing Mysterious Past, usually a Multiple-Choice Past.
Try to get under the hero's skin and break or corrupt them in some way.
It's worth noting that the Joker's design was largely inspired by Conrad Veidt's depiction of Gwynplaine, from the 1928 film adaptation of The Man Who Laughs. Unlike the Joker, however, Gwynplaine was a wholly good character who simply looked disturbing because of the Glasgow Grin he suffered from.
A Sub-Trope of Expy.
For his archnemesis see Batman Parody.
Practically Joker has been suggested to play 3 roles. Click below to see other actors suggested for each role, and vote for who you think would play the role best.