Screen Acting is as much about reacting, as it is about acting. (I think it is probably more, but there is a limit to the amount of aggravation I should cause you at this point.)
In real life, when a group of people are talking, their eyes tend to go toward the person who is talking at the moment. Often the person who wants to speak next tries to give visual signals (such as raising a finger, leaning forward with the body) to get the others' attention and so get to speak.
In stage acting, the audience spends most of its time watching the actor who is speaking. If another actor draws the eye during a speech by a fellow performer, it is called upstaging and is not usually welcomed. (I once directed a stage actor who would mop his brow vigorously with a green silk handkerchief during the funny lines of his fellow actors, preventing them from getting a laugh from the audience. I thought this was a little peculiar-and so would you, since they were all in a play written by the green handkerchief waver.)
On screen, when there are two actors on the screen, one talking and the other listening, I believe that the audience watches the listener more than the speaker. Look at the picture at the beginning of this chapter-you can see that it is the combination of the talker and the listeners that make it such a powerful image. This is quite logical, if you think about it, since we can tell from the sound of the voice more or less what is on the face of the speaker-