You learn something new everyday. More for your weekend viewing pleasure (see the below 2 posts).
I particularly enjoyed and appreciated this video conversation between Porgy & Bess 2012's immensely successful Broadway revival) understudies, Sumayya Ali and David Hughey (with host Caskey Hunsader for his Standing By blog).
In it, they discuss their careers and the challenges of being ready to go on at any moment in the Tony Award winning (Best Actress-Audra McDonald & Best Revival) production of Porgy And Bess, which played 322 performances in 2012, 17 more than the 1953 revival, making it the longest-running production of Porgy and Bess on Broadway thus far.
So what is an "understudy" you ask?
In theater, an understudy is a performer who learns the lines and blocking/choreography of a regular actor or actress in a play/musical/opera. Should the regular actor or actress be unable to appear on stage because of illness or emergencies, the understudy takes over the part. Usually, when the understudy takes over, the theater manager will make the announcement prior to the start of the performance.
That's the short answer. Experts in the field will also point out the differences between covering a part, swings, super swing, standbys, and alternates. But I'm not an expert; however, my research skills are unrivaled!
I should note that a number of actors actually made their name in show business by being understudies of leading actors, and taking roles over for several performances, including: Anthony Hopkins for Laurence Olivier, when Olivier became ill with cancer during the run of the National Theater's The Dance of Death, 1967; Ted Neeley for Jeff Fenholt during the 1971 Broadway run of Jesus Christ Superstar when Neeley was asked to star in the 1973 film version and subsequent tours; and more.
Watch Sumayya and David talk about the lives as understudies below (it gets more interesting as the conversation continues):