I challenged a former student, now long-time friend, Geoff Culbertson, to define Action. He’s what he sent me:

Geoff’s Definition of Action
“An action is one side of a dual interaction. This interaction is most often external: a person, an object, a place. However, it can be internal: an emotion, a thought, an image. (A complex line-of-action will involve the seamless use of all these elements.) An action is observable by an audience as a small unit of human experience that communicates a psycho-emotional state. Action is demonstrated through the delivery of a line, the expression of a gesture or some combination of both. An actor’s expression of an action is dependent upon the sense receptivity of the actor’s body and the image receptivity of the actor’s mind. The clearer the actor’s receptivity to images and senses, the greater the actor’s capacity for truthful action. The most difficult and most important aspect of teaching acting is making the student aware of the other side of an action: points of interaction. What Declan Donnellan refers to as the Target. If a student can relax enough to become aware of the shifting nature of awareness and surrender the instrument of the body to a receptive posture in relation to points of interaction – a natural flow of action will arise.”

If that’s somewhat too complex for you, here’s Earl Gister’s definition of action. Earl was a master teacher of acting, the mentor at the Yale School of Drama program for many years.

Earl’s Definition of Action – “Try to make the other actor really feel something.”

If you DO what Earl is suggesting, I think you will discover how wise and complete Geoff’s definition happens to be.