Understudy
If a main performer is unable to perform, the understudy will take their place. The understudy is often a member of the chorus who might have had a small part in the performance anyway.  Sometimes an understudy is called a "standby".  A standby is usually not a member of the chorus or ensemble.  They may be required to be present in the theatre during the performance, or part of it, in case of a problem.