How we cite our quotes: All quotations are from Vertigo.
Quote #1
ELSTER: Scottie, do you believe that someone out of the past, someone dead, can enter and take possession of a living being?
Elster's question is meant to be shocking both to Scottie and to the viewer. It's meant to jar us out of our comfort zone and get us to ask ourselves: "do we believe that?" It sets up the audience to believe we're going to see a story about spirit possession.
Quote #2
SCOTTIE: You'd better take her to the nearest psychiatrist, psychologist, neurologist, psychoanalyst, or plain family doctor. And have him check you both.
Scottie's a skeptic; he thinks there are simpler explanations for Madeleine's behavior and he thinks Elster is nuts for believing her. This exchange is the setup for a major plot development: Scottie's transformation from skeptic to believer.
Quote #3
MIDGE: And the idea is that the Beautiful Mad Carlotta has come back from the dead, to take possession of Elster's wife? Ah, Johnny! Come on!
These early, dismissive words of Midge's turn out to be even wiser than they sound the first time around. Only after we've seen Vertigo once do we realize how prophetic Midge's "Come on!" is. Right off the bat, Midge is presented as the grounded, sensible one.