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Jeff Clark--the guillotine represented his fear of Lang, who wanted to use Jeff's head for the creature.
Dr. Lang--the headless being represented the creature that he was building.
Julia--the skeleton bride represented her fear/hatred for the ghost of Josette, Barnabas's lost love.
Willie--the wolves, whose howling he always hated, represented his fear of vampire Barnabas.
David--the bats that he saw were a reminder of the time that Barnabas had menaced him.
Vicki--she was afraid of harm coming to Barnabas, and so she dreamed of him dead.
I'm not sure why Maggie saw the skull or why Carolyn saw her own gravestone (unless that's just something that generally frightens people.) I'm afraid I don't even remember what Mrs. Johnson or Sam Evans saw, so my analysis will have to end here.
ProfStokes