From the forthcoming book by Robert Epstein
From page 1:
This book also exists within a specific tradition, that of behavioral realism, and reflects the goals and principles of this school of acting. It stems from Konstantin Stanislavski's Method Of Physical Actions and Active Analysis, and strongly reflects the contributions of the major American teachers Sanford Meisner and Stella Adler.
Stanislavski's later work involved finding specific physical actions to create a road map of a role. These actions were found from the actor's truthful immersion in the imaginary life of the role, with full psychological involvement and physical commitment. "Truthful" acting and involvement, as used here, means that the actor's work is "real," that he is trying to live the part as if it were actually happening, and is responding authentically to the imaginary life of the role.
Through a long, involved series of active rehearsals, with emphasis on the "truth" of the role, a logical chain of actions would be discovered improvisationally and then gradually refined. Each external action would mark a place in the inner journey of the role, and provide concrete access to the role's inner life.
From page 3:
Meisner believed in the primary importance of specific behavioral responses. He believed that by going fully with behavioral action and interaction, working directly off of other actors and objects in the environment with full physical engagement, the actor would feel a real involvement in the imaginary situation and would begin to generate consistent truthful responses in the moment. His training method focused on the actor's response to each moment, pinpointing dramatic action's most basic unit: one clear stimulus, and one specific truthful response.