“龙套”是传统戏曲角色行当(type of roles in traditional Chinese operas)，指跟班、随从、兵卒等小角色(actor playing a walk-on part [e.g. attendant or solider])，因所穿的戏装为龙套衣而得名。

“跑龙套”原指在戏曲中扮演配角，后比喻做无关紧要的事(do things insignificant or trivial)，起次要作用(play a minor role)，或专做跑腿、服务性工作(be a utility man)。现在也指电影里面的群众演员(extras in a film)。

例句：

刚开始我跑跑龙套，后来我演上了主角。

I started as a walk-on, and later played principal roles.

我只不过是个跑龙套的，不拿事儿。

My job is to run errands, not to make decisions.