The Kuleshov effect is a film editing effect demonstrated by Soviet film-maker Lev Kuleshov in the 1910s and 1920s. It is a mental phenomenon by which viewers derive more meaning from the interaction of two sequential shots than from a single shot in isolation.
The effect is used when there is a shot of an expression on someones face then a shot of what they were looking at and finally their reaction to the second shot.
Although this was a film effect/technique it was originally an experiment, kuleshov used shots 1 and 3 (the characters face) several times but changed shot 2 each time. He used very different images for the second shot such as a new born baby to a murder scene. Kuleshov concluded that even if shot 1 and 3 are the same for each attempt, its shot 2 that determines the characters emotional thinking and intent.