ON ACTING: "Energy"
It comes from the same source as all other human energy: from a desire to accomplish a task.
Think of our everyday life; how often we feel tired, lackluster, out of sorts. We can't seem to summon up the energy to do anything. Then someone enters our sphere, our world, and mentions doing something that seems fum, engaging, important to us to accomplish. Energy seems to arise in our body. We rise out of the chair. Our conversation becomes sprightly. Our mind begins racing with ideas.
Goal is the tap root to human energy.
What happened to the old tiredness? Did it disappear? No; it has been replaced by a new flow of chemicals and electromagnetic activity within our bodies, like a dry riverbed suddenly being filled with swift flowing water headed toward a downstream goal.
When an actor feels listless, and his performance becomes correspondingly lackluster, one of the prime reasons often is that the actor-as-character is not doing anything important (to the character) in the scene. Attaining a goal is not critically important to the actor-as-character's flow of energy; or aliveness.
The quick actor's corrective to such de-energized listlessness (besides doing less partying, getting more sleep and ridding oneself of debilitating panic!) is for the actor is to (1) make sure he has defined his character's goal in the scene, (2) make sure that character's goal has emotional urgency to it (the goal is emotionally important--critical--for the actor-as-character to achieve; it is driven by deep and personal emotional need), and (3) the actor-as-character accepts that he can only attain that character goal by actively engaging himself in the scene to overcoming any resistance, human or otherwise, to his attaining that goal.