Kuleshov Effect

The Kuleshov Effect is a well-documented concept in film-making, discovered by Soviet film editor Lev Kuleshov in the 1920s. Kuleshov put a film together, showing the expression of an actor, edited together with a plate of soup, a dead woman, and a woman on a recliner. Audiences praised the subtle acting, showing an almost imperceptible expression of hunger, grief, or lust in turn. The reality, of course, is that the same clip of the actor's face was re-used, and the effect is created entirely by its superimposition with other images.

More generally, the Kuleshov Effect is the basis of Soviet montage cinema, and is used in many many films since. The idea is that, by editing different things together, it is possible to create meanings that didn't exist in either of the images put together - constructing 'sentences' and 'texts' out of film.

Film

 * Most films from the Soviet era prior to the forced implementation of 'Soviet Realism' in cinema by Stalin. These include venerable classics like The Battleship Potemkin and The Man with a Movie Camera
 * Though modern usage is not exclusively in this manner, it is useful for cases where the "actor" is inanimate. In 2001: A Space Odyssey, HAL "displays" a broad range of emotions through being an unreadable red camera lens.
 * Rear Window extensively uses this trope to spend whole scenes switching back and forth between Jimmy Stewart and what he sees through his window. In one sequence he stares out his window as the focus of the scene switches between several of his neighbors who have very different emotions in their scenarios. His only reaction is to ultimately raise his glass to one of them. Scramble the different window scenes, and the tone changes greatly.
 * Stewart actually complained that Hitchcock used the editing of the film in general to create a different performance than the one that was given.
 * C-3PO in the Star Wars films, most notably A New Hope which has his biggest role. His face is deliberately completely neutral and expressionless, and Anthony Daniels doesn't over-emote with his body to compensate; but he manages to be a convincing Woobie. R2-D2 is even more difficult, with nothing resembling a face and virtually no movement other than rolling around. Yet between Daniels' vocal performance and Ben Burtt's synthesis of R2's "voice", they manage to carry the original movie — we barely even see the human characters in the first reel.
 * Darth Vader arguably counts too, for the same reason. Helps if you're voiced by James Earl Jones.
 * The infamous shower scene from Psycho is often used as an example of this trope. After watching it, everyone immediately understands that Janet Leigh's character has been stabbed to death, but if you slow it down, only three frames actually show a knife piercing human flesh (this is fast enough to count as subliminal messaging). The audience's understanding of what has taken place comes entirely from the way the images and sound are arranged, not from the actual content.

Western Animation

 * Machinima takes advantage of this trope due to the inability to pose the characters' faces, or in some cases even see their faces at all.