For Hitchcock, “pure cinema” was born when Griffith's camera crossed the barrier of the proscenium. This transgression freed film to discover a natural subject in theater, in the interpenetration of theater and the world reflected in the familiar ambiguity of the English word “acting.” Theatricality is theater's natural condition; the candid or unself-conscious can be depicted, in theater, only by acting. But when Griffith's camera broke the barrier of the proscenium, it assumed the capacity, as a matter of course, to unmask theatricality. In movies, the camera routinely distinguishes between gestures and expressions that are candid and those that are staged.

Film's opposition between the theatrical and the nontheatrical is grounded in, and grounds, its conventional ways of presenting human beings in the world. Typically, the camera alternately frames its human subject within public and private spaces. The frame of the “objective” shot is a stage on which a human being performs, subject to view by others in his or her world. Within the frame of the reaction shot, the subject views the spectacle of the world, expresses a private reaction, and prepares the next venture into the public world.