The fact is most singers take at least some cover work, even if it's only making sure to be on hand for those performances in a run that they aren't actually contracted to sing themselves. Dolora Zajick, for example, sang Amneris in Aida in October and was the scheduled cover for the role in November. She herself had to leave in the middle of a performance on Oct. 26, leaving Malgorzata Walewska, her cover, to take over Act IV. Then on Nov. 12, Yvonne Naef, the new Amneris, also got sick in the middle of a performance, and Ms. Zajick had to step in and sing Act IV. In short, she sang the equivalent of one complete performance, but over two nights—and for two full fees.