Living moment-to-moment is essential to theatrical power. It keeps the actor alive, and requires that he leave the structure behind.
“When someone says, ‘Hey, I just walk in there and say the lines and go home,’ I always know that what that person is saying is ‘I don’t want to talk about the way I work.’” -- Jack Nicholson
At acting’s highest levels, actors follow a common structure, which reduces the work to its fundamental components. The actor must know what he wants, why he wants it, what's at stake if he doesn't get it, what are the relationships... Everything in the script must be personalized. Once the structure is in place, he doesn't have to think about it or work at it. So, he's free to be available to what's happening in front of him--in the immediate circumstance, free to really look, listen and absorb as in real life.