Supporting Actor
Supporting Role, Minor Role
If you're on armpit duty you're definitely not the lead. You're standing next to the lead's armpit, because you're the supporting actor. You're in a supporting role.
わきやく
The reading is 脇's reading and 役's reading, put together.
彼女、オーディションに受かったって言いふらしてるけど、ただの脇役みたいだよ。
She keeps talking about how she passed the audition, but it looks like she only got a supporting role.