In theatre, blocking is the precise staging of actors in order to facilitate the performance of a play, ballet, film or opera. Both 'blocking' and 'blocks' were applied were applied to stage and theatre as early as 1961. The terms derives from the practice of 19th-century theatre directors such as Sir W. S. Gilbert who worked out the staging of a scene on a miniature stage using blocks to represent each of the actors.
In contemporary theatre, the director usually determines blocking during the rehearsal, telling actors where they should stand for the proper dramatic effect, ensure sight lines for the audience and work with the lighting design of the scene. Each scene in a play is usually blocked as a unit, after which the director will move on to the next scene. The positioning of actors on stage in one scene will usually affect the possibilities for subsequent positioning unless the stage is cleared between scene.
During the blocking rehearsal, the assistant director, stage manager or director takes notes about where the actors are positioned and their movement on stage. it is especially important for the stage manager to note the actors' positions, as a director isn't usually present for all performances, it is the responsibility of the stage manager to ensure that the actors respect the blocking each night of performance.
In film, the term blocking is sometimes used to speak of the arrangement of the actors in the frame. in this context there's also a need to consider the movement of the camera as part of the blocking process.