There's some reason behind my choosing the still from Vertigo (incidentally one of my favorite movies) as the visual motto for this blog: it embodies so many forms of unreality all at once.
Let's begin with the directly visible elements in the image: we're in an art gallery, looking directly at a portrait, that's already one form of unreality; clearly, the woman sitting in front of the picture has drawn up some parallel between her and the portrayed lady (never mind Henry), as we can see from the bouquet of flowers which is identical in style. Of course we know that in the movie, Kim Novak's character has modeled herself after the depicted woman. This game of imitation (another form) leads to the complicated deception that's going on (yet another one): the murder plot, devised by a businessman who wants to kill his wife and needs a reliable and believable witness who would affirm the death to have been a terrible accident. To trick that witness (James Stewart) into believing in the accident version, the murderer and his accomplice go a long way with their deception, building up a fake personality. The scene we're witnessing in the art gallery is part of that construction. Moreover, it's in particular the idea of a person haunted by a ghost from the past (more forms of unreality: ghosts and the past) that is planted in Stewart's character. (Among other things supported from a story told by an old bookseller, mixing gossip, history- and storytelling into one.) One of the most extraordinary passages is a surreal dream sequence (one more).
Of course, all these are kinds of unreality internal to the film. The movie itself, as such, is another form, as is the novel (D'entre les morts, by Pierre Boileau and Thomas Narcejac) on which it is based. It has itself served as inspiration for further films: Chris Marker's La Jetee, which then was in turn the inspiration for Terry Gilliam's Twelve Monkeys, both of them exploring the idea of the accessibility of the past and the future, which are again forms of unreality. And finally, I think at least part of the dramatic conception relies on the fact that unreality can sediment into something that is no less a potent force in the world than whatever reality there might have been antecedently (as I have pointed out in my interpretation sketch in my previous post): the movie also reflects on unreality.
None of all these forms of unreality are accidentally there; they're all deliberately chosen and used with excellent skill. I think this contributes immensely to the density and beauty of Vertigo: it's at least as much about unreality as it is about the depths of the human psyche (if indeed that is a distinction that has a real difference behind it).