One of the most underrated aspects of Hitchcock's film is how it details the psychology of Marion Crane. The film is known for her annihilation. Yet, her shifting identity throughout the first part of the film is one of the reasons why it can be said that Hitchcock understood human psychology in the composition of his work.
Marion Crane is fundamentally dissatisfied with her life. She wants more than what is present. She wants a life with Sam, and is tired of the obstacles that prevent that from happening. In a larger sense, Marion is agitated at her condition in the world. This restlessness helps to fuel the shift in her identity. She is overcome by the client who "buys off" unhappiness with such a large amount of cash on her desk. Marion's shifting identity is reflected in the collision between what she wishes to do and what actually can be done. She takes the money for this reason and seeks escape because of it. It is for this reason that her mind replays the potential conversations that others have about her. She is "trapped," to quote what she and Norman speak before her fateful shower. She is "trapped" between this reality of what she wants to do and what is presented in front of her. It is for this reason that her identity shifts back and forth, causing her to take a night of refuge at the Bates Motel. When she confesses to Norman that she has to "remove herself from one of those traps," Marion's identity shifts to another point from which no other shift will be evident.