understudy

To study or know (a role) so as to be able to replace the regular performer when required.

To act as an understudy to.

To be engaged in studying a role so as to be able to replace the regular performer when required.

A performer who understudies.

A person trained to do the work of another.

To memorize (a part) as an understudy.

Theat., one who has made a special study of a particular part, and is capable of playing that part at a moment’s notice in the absence of the actor or actress to whom it is usually assigned.

One who studies another’s part with a view to assuming it in an emergency.

To study, as another actor’s part, in order to be his substitute in an emergency; to study another actor’s part.

to study or know a role to such an extent as to be able to replace the normal performer when required