The Kuleshov Effect...

The Kuleshov effect is an editing technique first introduced by Russian film maker Lev Kuleshov in the early 20th Century. It describes how simple editing of sequences in a clip can have an overwhelming effect on the characters and/or storyline. The way it works is simple, and a characters personality can be easily manipulated depending on the sequence. For example, if a young girl is shown looking at something, then a clip of a puppy chasing its tale is inserted, followed by a reaction shot of the girl smiling and laughing, the audience can infer that the young girl is an innocent animal lover. However, replace the clip of the puppy chasing its tale with a man getting seriously wounded but keep the reaction shot of the girl exactly how it was before, the young girl is suddenly percieved as heartless and evil.

Hitchcock was a huge fan of the Kuleshov effect and demonstrated it often in his work. Here is Hitchcock explaining it in simple terms:

-KW