Brown wowed audience members after stepping into the role of Eva Perón midway through a performance of "Evita" in London's West End on July 24.

The moment captured what theater lovers know but audiences don't always see: understudies and swings must be ready at a moment's notice.

Rachel Zegler had to exit stage left on Thursday. The actress, 24, who is starring in “Evita” at the London Palladium on the West End, abruptly left the evening performance due to illness. According to People, Zegler’s departure was announced during intermission.