跑龙套 pǎolóngtào to play a small role

怎么记住“跑龙套”: 跑 run + 龙 dragon + 套 case = to play a small role.

汉字

To play a bit part / be an extra - originally in opera and film - 他早年只能跑龙套 (tā zǎonián zhǐ néng pǎolóngtào) in his early years he could only get walk-on roles.
Figuratively, to do minor, unimportant work in any group (打杂跑龙套 running errands and playing second fiddle).
Colloquial and slightly self-deprecating; 龙套 (the costumed extras) is what you're "running," so treat the whole phrase as one verb.