a standby traditionally is someone of some name recognition who does not have a part in the show, until the "star" name is out. For instance, Gretchen Wyler stoodby for Lauren Bacall in APPLAUSE, Bibi Osterwald stoodby for HELLO DOLLY and Thelma Carptener for Pearl Bailey's Dolly.
Simply put, in the past anyway, a standby was someone with some name recognition, who would fill in for the star and hopefully the standby's own name recognition would help keep people from wanting refunds at the box office.
At a very young age, Elaine Stritch was the standby for Ethel Merman in CALL ME MADAM. Stritch talks about it in her one woman show..........
In more recent years.....the "title" of standby has been given to some people who were "actually understudies," meaning that they performed a "smaller part" in the show and "went on" for a leading player who was out. Normally, this performer would be called an understudy.
Apparently, the terms have had some variance in usage in the last few decades. Some folks here have mentioned people being called standbys in shows where the "standby" was performing another part. In strict theater parlance, this would be an understudy. However, I suspect certain folks have "negotiated" and perhaps get paid a bit more when performing a smaller part in the show and "understudying" a lead and have also negotiated to be called a "standby" for the lead as opposed to understudy. :)
"Blow out the candles Robert and make a wish. Want something, want SOMETHING."