Kuleshov Effect

http://en.wikipedia.org/wiki/Kuleshov_Effect
Kuleshov used the experiment to indicate how important film editing is.
"The implication is that viewers brought their own emotional reactions to this sequence of images, and then moreover attributed those reactions to the actor, investing his impassive face with their own feelings."

And I found a video that Hitchcock explains the Kuleshov Effect in 1964.