Originally a 京剧 term: 龙套 are the four or eight attendants in dragon-embroidered robes who follow a king or general on stage with no speaking lines. The modern verb 跑龙套 ('run as a dragon-robe') now means to play an insignificant or uncredited role.
Hong Kong comedy star whose 1996 film 《喜剧之王》 (King of Comedy) dramatizes a struggling 龙套 actor — the film made 'running 龙套' a cultural shorthand for paying one's dues in showbiz.