Understudy (noun)

1. An actor who learns the lines and stage business of a leading actor in case they are needed to replace them.
2. Someone who is trained to take the place of someone else.

Origin:

Mid 19th century: from under- + study.

Examples:

1. He was the understudy for the lead role in the play.
2. She was the understudy for the star of the show.
3. He was the understudy for the director in case of an emergency.
4. She was the understudy for the CEO in case of an absence.
5. He was the understudy for the manager, and he was ready to step in at a moment's notice.