Blocking is one of the major components of the rehearsal process. It is the time when the positioning and movement of the actors is decided by the Director.
Essentially, it is the manner by which the "choreography" of each scene is put down on paper...eventually to be put to memory by the actors. It is in this fashion that each Director is able to bring to life their vision of how each scene should unfold.
Usually, each scene is approached as a unit. During the course of the scene, actors take notes about their own physical movement as well as their relationship in space to their fellow actors, the furniture props, their personal props, entrances and exits, their position on the stage, etc. These instructions are what they pencil into their scripts for study and reference purposes. (I emphasize the pencil because rented scripts must be returned without marks and therefore anything written into them must be erasable.)
The Stage Manager in turn keeps a record of the movement in each scene by utilizing what is known in the theater industry as a "Prompt Book". The notation of movement is a technical process and it is a very important part of a Stage Manager's job. It has added importance because, especially in Professional Theater, the Director is not present during most of the run of a show. The Stage Manager is in charge and if there is a question about movement, the Prompt Book is law. Also, an actor may leave a show and the written record allows a replacement to easily transition into a role.
It is vital to the success of a show that any changes in movement made by the Director during the rehearsal process be accurately recorded in the Prompt Book. Actors must also note changes in their own scripts.
Keep an eye out for blocking changes during "Notes" given by the Director during the rehearsal period.
N.B. Always remember to change the notations you make for yourself during the rehearsal period. Directors change their mind and it is your responsibility to make the changes. Additionally, it is your job as an actor to portray your character in the best way you can according to the Director's wishes.
From personal experience in Stage Management I have found one book to be great for the novice. If you want to learn more about how a play is stage managed I recommend taking a look here
at this book.
at this book.