Final answer:
The area behind the curtain where the actor cannot be seen is called backstage or offstage. These areas are essential for various aspects of a theatre production, including costume changes and stage management.
Step-by-step explanation:
The area behind the curtain where the actor cannot be seen is known as backstage or offstage. In a traditional proscenium theatre, this includes spaces like the green room, which is a lounge-like area for actors to relax in between scenes, and dressing rooms, where actors prepare their costumes and makeup. Technical spaces like the booth for the stage manager and technicians are also considered offstage areas.
However, in an arena stage setup, the nature of the stage itself means that it's more challenging to hide activity from the audience, as the action happens in the round with the audience encircling the stage. Nonetheless, backstage areas are still critical for the operation and execution of a theatre production.