Acting is all about how you feel. It is about experiencing the emotional response at the time you feel it and how that comes through and communicates to the people watching your art. This comes from finding moments. Moments of truth, emotion, natural response. This only comes from being in the present and focusing on whomever you are working with. You experience them and get their emotions in the moment and it reacts within you and forces out a certain reaction. The truth in this, and in living in the moment, comes from the personal experience each and every time you have this exact reaction. This is why the great actors can do a scene in a play a million times and still have it ring true; they are doing it anew in a new moment every single time.
      A great acting coach of mine once compared acting to life, he said that the reason people want to go to the movies, see a show, or watch something on TV is to experience something outside of their normal life. Something that may not have anything to do with them, but something that they can relate to. They want to experience that. So in order to make someone else experience something you need to experience it yourself. This is what you achieve when you are really living in the moment and able to react off of the things your scene partner is doing and vice versa.
      If you are really living as the character, there will never be a time when the performance is exactly the same as the last. This is because everything will affect you as it happens. The way a character looks at you may change all of your feelings about them, it may change the way you exit the scene, it also can change how you come into the next scene which will, in turn, affect your scene partner. In life we are always moving quickly, often times not stopping to really experience and feel whatever it is we are going through. In acting, you can afford to give yourself the time, so enjoy it!