(This is pretty long, so I've bolded the parts that cut to the heart of the matter, for easy skimming.)
From David Mamet's True and False: Heresy and Common Sense for the Actor:
My philosophical bent and thirty years' experience inform me that nothing in the world is less interesting than an actor on the stage involved in his or her own emotions. The very act of striving to create an emotional state takes one out of the play. It is the ultimate self-consciousness, and though it may be self-consciousness in the service of an ideal, it is not less boring for that.
The actor on the stage, looking for or striving to create a "state" in himself can think only of one of two things: (a) I have not reached the required state yet; I am deficient and must try harder; or (b) I have reached the required state, how proficient I am! (at which point the mind, ever jealous of its preogatives,will reduce the actor to (a).
Both (a) and (b) take the actor right out of the play. For the mind cannot be forced. It can be suggested, but it cannot be forced. An actor onstage can no more act upon the order "Be happy" than she can act upon the order "Do not think of a hippopotamus."
Our emotional-psychological makeup is such that our only response to an order to think or feel anything is rebellion. Think of the times someone suggested that you "cheer up," of the perfect young person your friends wanted to fix you up with, of the director who suggested you "relax." ...If one were truly able to command one's conscious thoughts, to summon emotion at will, there would be no neurosis, no psychosis, no psychoanalysis, no sadness.
...The skill of acting is finally a physical skill; it is not a mental exercise, and has nothing whatever to do with the ability to pass a test.
The skill of acting is not the paint-by-numbers ability to amalgamate emotional oases--to string them like pearls into a performance (the Method). Nor is it the mastery of syntax (the academic public speaking model). The skill of acting is like the skill of sport, which is a physical event. And like that endeavor, its difficulty consists to a large extent in being much simpler than it seems. Like sports, the study of acting consists in the main of getting out of ones' own way, and in learning to deal with uncertainty and being comfortable being uncomfortable....
The opportunity for bravery is always there--it is always in the play itself.
Let me explain. The actor says to himself, "I can't play this scene because I am unprepared; I can't play it because I don't like the other actor, who is a swine; I feel that the moment is wrong as the director has interpreted it; I feels this flies in the face of my preparation; the script isn't as good as I thought it was," and so on.
All of these feelings are engendered by the script and they are always and only engendered by the script...and all our excuses, all those supposed "impediments" to acting are, if we listen closely, merely the play asserting itself. The actor creates excuses not to act and attributes her reluctance to everything in the world except the actual cause. The play itself has brought her to life in ways she had not foreseen, and she doesn't like it one small bit... ..Acting...requires not tidiness, not paint-by-numbers intellectuality, but immediacy and courage......
In life there is no emotional preparation for loss, grief, surprise, betrayal, discovery; and there is none onstage either....
...When the performance is made truthful, the work of the writer is made something more than words on the page, not by the inventiveness but by the courage of the actor. Yes, it might seem like a good, and might seem like an attractive idea to embellish--it's your job to resist that attractive idea...Invent nothing. Deny nothing. Develop that hard habit.
...The addition of "emotion" to a situation which does not organically create it is a lie. First of all, it is not emotion. It is a counterfeit of emotion, and it is cheap...The greatest performances are seldom noticed. Why? Because they do not draw attention to themselve, and do not seek to--like any real heroism, they are simple and unassuming, and seem to be a natural and inevitable outgrowth of the actor.
...Any system built on belief functions through the operations of guilt and hypocrisy. Such a system, whether of acting training, meditation, self-improvement, etc., functions as a pseudo-religion...The system holds itself out as the alleviator, cleanser, and redeemer of the guilty individual.
Now, none of us is free of self-doubt, and none of is free of guilt. We all have thoughts, feelings, episdoes, and tendencies which we would rather did not exist.
...Curiously, the state these systems profess to cure--anxiety, guilt, nervousness, self-consciousness, ambivalence--is the human condition...and, coincidentally, the stuff of art....
You went into the theatre to get an explanation. That is why everyone goes into the theatre...Your fear, your self-doubt, your vast confusion (you are facing an ancient mystery--drama--of course you're confused) do not mar you.
...Respect for the audience is the foundation of all legitimate actor training--speak up, speak clearly, open yourself out, relax your body, find a simple objective....The actor before the curtain, the soldier going into combat, the fighter into the arena, the athlete before the event, may have feelings of self-doubt, fear, or panic. These feelings will or will not appear, and no amount of "work on the self" can eradicate them.
The rational individual will, when the bell rings, go out there anyway to do the job she said she was going to do. This is called courage.
...Acting, like any art, can be learned, finally, only in the arena.
One can read all one wants, and spend eternity in front of a blackboard with a tutor, but one is not going to learn to swim until one gets in the water--at which point the only "theory" which is going to be useful is that which keeps one's head up. Just so with acting...
...Acting is a physical art. It is close to the study of dance or of singing. It is not like the study of mechanical drawing or literature....
Let the politicians have their fixed smiles and their crocodile tears, let them be the unabashed promoters of their own capacity to feel. Let us be circumspect and say the words as simply as possible, in an attempt to accomplish a goal like that delineated by the author--and then both our successes and our failures can have dignity.