Opera prompters are traditionally housed in a stuffy wooden box at the center-front edge of the stage, above the orchestra pit. They are visible to the performers and no one else. Technology has brought cool air and small display screens, among other advances, to support their work.
Effective prompting can be a challenge. The American prompter Philip Eisenberg recounts the story at a Maria Callas performance when she needed louder prompts. The famed diva swooped down in a curtsy right in front of the prompter’s box and — mid-curtsy, unnoticed by the audience — gave the Italian command “più forte!” (louder) to her boxed colleague.
Prompters attend all rehearsals, mark up any adjustments or clarifications to the score, and generally “prepare” singers for a role. Their profile is low, and opera program books often credit them only under “musical preparation” or some similar moniker.