Although a somewhat fluid kind of work, in essence the stage management team (in large productions, often consisting of a production stage manager, several assistant stage managers, and any number of production assistants) is responsible for organizing the production, communicating across different diciplines (e.g. between the director and the backstage crew, or the actors and production management), and keeping everything running smoothly. The latter may consist of tempering a volitile director, soothing a harried actor's nerves, or simply handing out candy.
The responsibilities and duties of stage management vary depending on the setting of a production, ie rehearsals or performance, and the type of production being presented (theater, dance, music). Typically in theater, the stage manager acts as an adjunct to the director in rehearsal, recording the staging directions and seeing that cast members stay on script, have necessary props, and follow the staging instructions. As the lighting and sound cues are developed, the stage manager meticulously records the timing of each as it relates to the script and other aspects of the performance. Once the house opens, the stage manager essentially takes control, calling the cues for all transitions, as well as acting as communications hub for the cast and crew.
Professional stage managers in the United States are represented by Actor's Equity and have more prescribed responsibilities. They are also responsible for maintaining the schedule and rehearsal hall within the guidelines of the Equity agreement. See also: stagecraft, theatre,