In the 1920's, Russian film makers Pudovkin and Kuleshov proved this principle. They alternated a MS of a man staring blankly into the camera with a CU of a bowl of soup; a MS of a dead woman in a coffin; and CU of a little girl playing.
To viewers, the man's blank expression suddenly mirrored the emotions in the other images to the point of creating an imagined cause and effect be-tween them.
Although footage of the actor's face was the same, when the coffin footage came first, the audience thought the actor showed deep sorrow. When his face followed a CU of the soup, the audience thought he was hungry. And when he was edited to the shot of the little girl playing, the audience saw the actor feeling parental pride.
The footage of the actor's face has no real connection with the other shots, except in the viewer's imagination when seen in sequence. We mentally "project" our responses to the images onto the actor and believe we know what he is thinking or feeling.