Beneath the stage of Broadway’s Al Hirschfeld Theater lies a dark space with a low ceiling — a cross between a suburban rec room and a submarine. Shortly before each performance of “Kinky Boots,” 13 black-clad musicians file to their battle stations, each illuminated by a small light attached to a music stand. The expectant buzz of the audience filters down from a narrow opening at the edge of the stage.

As at any Broadway show, the musicians in the “Kinky Boots” pit are expected to play flawlessly for two-plus hours — even those who are sitting in for what may be the first time. During a recent Sunday matinee of the hit show, for instance, four of the musicians were substitutes, called in and asked to unobtrusively join a band grooving at the top of its game. After all, even Broadway musicians may want to take vacations, or spend the summer touring with Sting or the Rolling Stones.

“Subbing is a bit nerve-racking,” said Ann Klein, 52, who replaced Michael Aarons, the regular “Kinky Boots” guitarist, twice in July. Ms. Klein has worked as a replacement on Broadway for five years, in five different shows. She has a career as a singer-songwriter and will pick up work on tour with other artists. But substituting on Broadway pays pretty well — like regulars, subs earn union scale, $227.42 per performance — if you can deal with the high stakes. “You don’t have the luxury of rehearsing with the band,” Ms. Klein said. “So it’s scary.”

“Once,” Jeff Schiller, another “Kinky Boots” sub, recalled, “I got a call half an hour into a show, when a regular was experiencing incredible kidney stone pain.” Luckily, Mr. Schiller, who goes by the nickname Houndog, lives near the theater district. He swapped in between numbers in the middle of Act One.