S. taught that the actor must have exact knowledge of himself and of the people he sets out to portray. Nothing that is not taken from the actor's observation, or confirmed by observation, is fit to be observed by the audience.
— Bertolt Brecht, 1952. 
“An actor should be observant not only on the stage, but also in real life. He should concentrate with all his being on whatever attracts his attention. He should look at an object, not as any absent-minded passer-by, but with penetration. Otherwise his whole creative method will prove lopsided and bear no relation to life.”
Stanislavski noted that powers of observation were not equally distributed: some people are naturally observant; others “are unable to develop this power of observation even sufficiently to preserve their own interests…. But you cannot put into a person what he does not possess; he can only try to develop whatever power he may have."
He encouraged actors to study nature, then human creations (art, literature, and music), and finally human behavior. Here, Stanislavski believed rational analysis was inadequate. “Very often we cannot come through definite data to know the inner life of the person we are studying, and can only reach towards it by means of intuitive feeling…. Here … we are dealing with powers of observation that are subconscious in their origin.
Brecht, too, believed that an actor’s performance should demonstrate his or her knowledge of human behavior. Going beyond Stanislavski’s insistence that an actor not try to arouse a feeling directly, but focus instead on actions, Brecht was less concerned that an actor make his character understandable to the audience than that he or she communicate what takes place.
“Observation is a major part of acting,” Brecht wrote. “The actor observes his fellowmen with all his nerves and muscles in an act of imitation which is at the same time a process of the mind. For pure imitation would only bring out what had been observed; and this is not enough, because the original says what it has to say with too subdued a voice. To achieve a character rather than a caricature, the actor looks at people as though they were playing him their actions, in other words as though they were advising him to give their actions careful consideration.”
© H. Clark Kee, 2009