Assistant Stage Managers are appointed by the Stage Manager to assist with specific jobs for the run of a show. Such jobs might be running all scene changes and co-ordinating all stage crew from one of the two wings, meaning that the Stage Manager does not have to split their time between both sides of the stage; being responsible for props on one or both sides of the stage; or controlling one specific piece of complex scenery or special effect, so that the Stage Manager is not overly focussed on one effect and can ensure that there are no unexpected problems elsewhere.

Assistant Stage Managers are distinct from Stage Crew in that they are generally expected to be present for all performances, so that any such delegation of jobs from the Stage Manager can happen safely and without additional risk to the cast or other crew members. However, some longer-running shows offer ASM roles on a rota basis or request additional help on just a few nights.