The phrase "psychology in film" evokes images of dark, decaying insane asylums, filled with dark and decaying minds. Patients crying out in the night, hearing voices, and seeing people who aren't really there. Undoubtedly, this is a form of psychology. However, psychology often presents itself in more subtle ways. Movies are a reflection of the real world. In order for characters and stories to be believable, they must live by the same psychological rules that govern our everyday lives. As a result, we can use film as a window into the human psyche.
The most important thing to understand about psychology is that the line that we draw between "normal" and "abnormal" is not a clear one. Rather, psychological functioning operates on a spectrum between the two extremes. All "abnormalities" result from "normal" brain features that are either over or under functioning. Before we can even attempt to understand what happens when these features go haywire, we must understand what is in the realm of normalcy. Film can be used to help us do this.