Swing
A swing is a performer who understudies multiple roles in a theater production, stepping into various parts when needed to keep the show running smoothly.
Understudy
An understudy is an actor who learns a role and is ready to step in if the main actor cannot perform, ensuring the show goes on.
Standby
A Standby is a performer specifically contracted to cover one or two named leading roles in a theatrical production — remaining on call and prepared but not performing in the show unless needed.