The prompt (sometimes prompter) in a theatre is traditionally the person who prompts or cues actors when they forget their lines or neglect to move on the stage to where they are supposed to be situated.
Nowadays, many of the earlier duties of the prompter are undertaken by the stage manager, who will have a copy of the script called the prompt book. This is the most definitive version of the script for any one performance, and will contain details of all cues, with their precise timings with respect to the action on stage.  This allows the prompt to direct lighting, sound, flying effects and scene changes during a show. The prompt book also often contains blocking notes, so that the prompt is always aware of the intended positions and movements of all the actors on stage at any given time.
In professional and most high-quality community theatre productions, the prompt is never used during a performance to instruct actors if they forget a line or movement, only during a rehearsal. If prompting is absolutely necessary, it can be done very quietly by another actor on-stage.
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