Unthinking people speak of the motion picture as the medium of "action"; the truth is that the stage is the medium of action while the screen is the medium of reaction. It is through identification with the person acted upon on the screen, and not with the person acting, that the film builds up its oscillating power with the audience. This is understood instinctively by expert film-makers, but to my knowledge it has never been formulated. At any emotional crisis in a film, when a character is saying something which profoundly affects another, it is to this second character that the camera instinctively roves, perhaps in close-up; and it is then that the hearts of the audience quiver and open in release, or rock with laughter or shrink with pain, leap to the screen and back again in swift-growing vibrations. The great actors of the stage are actors; of the screen, re-actors.