There are three phases to an actor's performance: Listening (or observing the actions of another character), reacting to what has been said or done by another actor and, finally, speaking your own dialogue or performing an action. The clips here from two of my movies show examples of responsive reactions; an actor reacting to the dialogue of another actor. The reaction can be long and articulated or short and to the point. In either case, it is performed after the other actor speaks and before you speak your own dialogue.
This take is from a scene from my movie Exigence. It is a good example of "responsive reactions" per my Action/ReAction technique for actors. A responsive reaction is performed a half beat after the other actor concludes his or her speech or action (for example, if he pulls a gun on you or waves 'hello') and before you say your dialogue.
This scene from my movie Point of Departure shows responsive reactions in a completed scene. Notice that I purposely removed her reaction before she says "Why is that?" and again before ""Where is he now?" so the next reaction would have more force. In the next two exchanges, using the responsive reactions escalated the emotional tension and amplified the impact of the scene.
A responsive reaction can be subtle and of very short duration or it can be profound and lengthy conveying multiple unspoken thoughts to the viewer (watch some of Marlon Brando's performances as he was brilliant at this)--it is up to the actor to orchestrate his or her performance. The responsive reaction is, I believe, the most important phase of the performance because it allows an actor to communicate something to the audience that isn't contained in the dialogue.
In my feature film The Dearly Departed, Debbie Hartner's character receives some astonishing news from a stranger who has come to her door. The magnitude of her responsive reaction tells us all we need to know.
In this segment from my TV series Clip Joint, notice how J.T.'s reactions escalate as the character fails to get his instructions correctly.