Schtick

A Schtick or shtick is a Yiddish expression which refers to a comic theme.

The term is best explained by giving examples:

Famous Comedians And Their Schticks

Jack Benny's character on his radio program was notoriously 1) stingy and 2) a bad violin player. In real life, Benny was known as an expert violinist and lavish tipper.
The Blues Brothers, the dark-suited alter egos of Dan Aykroyd and John Belushi, began as a schtick and grew into a record and two movies.
Johnny Carson's schticks include Karnak the Magnificent, an Indian fortune teller who could divine answers to questions sealed in envelopes and kept in a mayonnaise jar on the front porch of Funk & Wagnalls since earlier that afternoon.
Andrew Dice Clay is a crude misogynist (at least one hopes this is just his schtick).
Rodney Dangerfield don't get no respect.
Jerry Lewis's schtick was to act like a schlemiel, but in real life (for example as the host of the annual Muscular Dystrophy Assocation telethons) he is generally regarded as a mensch.