Quote From Lee Strasberg
"Once the actor begins to think, life starts, and then there cannot be imitation."
To read more of his quote:
"Stanislavski was the first to realize that the cliche, the conventional idea idea of what is to be accomplished in a scene, satisfies a a very strong need both in the human being and in the actor; that the cliche functions as an habitual response; that it is at best a caricature of what really happens in life; and that once the habitual response is interrupted by something the actor does not expect, the cliche vanishes because the actor really has tothink on stage."
Lee Strasberg in 'Strasberg at the Actor's Studio: Tape-Recorded Sessions'.