Guide for the Perplexed

I don't think it would be in the spirit of the film to completely spell everything out. But I will provide just enough that someone who really wants to get it can go back and watch the movie while following the threads I mentioned and start to tune into the darker meanings underneath the surface.

The back-story - Eleanore and Max fell in love and got engaged. To her dismay, Eleanore discovered that Max was impotent. Eleanore began a passionate affair with Carlo, Max's stepfather. They planned to elope. Max caught them making love and murdered them both with a cane in a fit of rage. The Countess was devastated by the discovery of her husband's treachery and her son's crime. In order to protect her son, the two withdrew and kept themselves confined to the villa. Here, they are perpetually performing ceremonies over their lost loves. The Countess buries an effigy of her husband each year to commemorate his death. Max has hidden the body of Eleanore in his room. He keeps her on the bed and brings her offerings of chocolate cake (with sprinkles, of course). (Offerings of small cakes to the spirits of the dead is an ancient custom in many religions. This is a darkly satirical take on that and a reference to their wedding cake.) Max spends the rest of his life perennially staging his wedding night - attempting and failing to make love with Eleanor, while her ghost mocks him in his mind.

1. The music box is the primary key. It is a mechanical Danse Macabre in which Death leads his victims in a circle instead of a line. Pay attention to the circles. They're in almost every scene. Even the camera moves in circles. Also, the music box is connected to the car, the bus, and the airplane (Notice the airplane makes the noise the car made when it broke down, and Leandro has a recording of the car horn in the music box). Again, mechanical travels that break down and don't go anywhere.
2. That's not paint on the dummies' heads. You are seeing the effects of a scene that takes place later. (In other words, you are seeing the effects of an event that has already happened, but you will see a re-enactment of the event later.)
3. Notice how Bava visually confuses Lisa and Sophia. Why does Max kill George? Why does he kill Sophia with the cane?
4. The Ceremony is the other key. Ceremonies are circular. We repeat them the same way, again and again. We repeat them because we have to. Ceremony destroys identity - it doesn't matter who you are or what you feel, as long as you play the part that the people around you need you to play. This is the secret tragedy of the film, and the heartbreaking truth about who Lisa is and why she can't be Lisa, why Eleanor could not be liberated by death.

If you listen closely to his speeches, Leandro makes it clear that he is as much a slave of the ritual as the others. He must play the role of the Devil because they NEED him. He is weary of the role. They are all trapped in an eternal ceremony that is completely of their own making and perpetuation. Eleanor tries to escape the ceremony by becoming Lisa. Leandro has to reclaim her because she's part of the menagerie - she cannot have a life of her own because the others need her. She and Carlo must come back.