noun
substitute; stand-in; understudy; double
Refers to a person who takes over a role, especially in theater, film, or performance, but also used more broadly for any temporary replacement.
主演俳優が病気で、代役が舞台に立った。
The lead actor was ill, so the understudy went on stage.
急な欠員が出たので、彼が代役を務めた。
When someone suddenly couldn't make it, he filled in as a substitute.