In theatre, blocking is the precise staging of actors to facilitate the performance of a play, ballet, film or opera. Historically, the expectations of staging/blocking have changed substantially over time in Western theater. Prior to the movements toward "realism" that occurred in the 19th century, most staging used a "tableau" approach, in which a stage picture was established whenever characters entered or left the stage, ensuring that leading performers were always shown to their best advantage. In more recent times, while nothing has changed about showing leading performers to their best advantage, there have been changing cultural expectations that have made blocking/staging more complicated. There are also artistic reasons why blocking can be crucial. Through careful use of positioning on the stage, a director or performer can establish or change the significance of a scene. Different artistic principles can inform blocking, including minimalism and naturalism.