Being in the moment
It has forced me to become a better listener not only on the stage but in life. I've learned to listen for any and all information that might help me form a better understanding of the character I'm talking to.
When I'm firing on all cylinders during a scene everything falls away except the characters on stage. I'm present as my character, taking in information and reacting spontaneously. When I say something and have no idea afterward where it even came from that is how I know I'm in the moment.