The Madeleine character of Vertigo is a fabrication from the start, a fact that is not known until two-thirds of the way into the film when it is revealed that Judy impersonated Madeleine in a scheme to murder the real Madeleine Elster. It is a fact that unmoors viewers as it means that Madeleines apparent motivations, haunted dreams, memories, and even mannerisms have been externally created by Judy in collaboration with Elster. Madeleine is the perfect representation of the world of romantic illusion to which Scottie is tragically attracted. It is difficult to discus s what motivates Madeleine because she is no more than a projection. Judy, on the other hand, is a real person, complete with imperfections, complex feelings, and motivations. Where Madeleine represents the unattainable ideal, Judy represents the real. The only point at which Judy and Madeleine converge is in their love for Scottie.
Judys manners are unrefined, even a bit coarse. In short, she is the antithesis of the refined, ethereal Madeleine. But Scottie recognizes some echo of Madeleine in Judy and relentlessly quizzes her about her identity. At first, Judy defends her true self, repeating her name, the name of her hometown in Kansas, and her occupation. In retrospect, we see that she is probably desperate to reclaim her true identity after having played the role of Madeleine for so long. When it becomes clear to Judy that Scottie will never love her for her own attributes, she consciously surrenders herself and allows him to transform her into Madeleine. Indeed, by the time her transformation is complete, it seems that rather than playing a role, Judy has actually taken on Madeleines identity, a fact that would account for her unthinking and fatal choice of Carlottas necklace when she dresses for dinner.
In Hitchcock's Vertigo, Judy must die for the sake of narrative resolution as, while Scottie desires her, she disturbs his need for mastery. Madeleine often appears through a doorway, indicating that theres something behind her, in her past that is only partially visible to us, and to Scottie.