If you have a look to your left, at an ad from Variety- from October 17th 1979, you'll see that the proposed cast list for MANIAC, as it went into production, is pretty different to the cast that actually ended up in the film.
Daria Nicoldi was, at the time, Mrs Dario Argento (and is the Mother of Asia Argento); she was the star of quite a few Italian genre films- including Mario Bava's SHOCK (aka BEYOND THE DOOR 2) (1977) and Dario Argento's DEEP RED (1975). MANIAC was all set to be her American debut- she was due to take the role that later went to Caroline Munro. Legend has it that Nicoldi, who was no stranger to screen blood splashes, was so shocked by the level of violence in Lustig's film that she backed out- subsequently never having her breakthrough film in the States.
Susan Tyrell, who was great as the psychopathic Aunt in NIGHTMARE MAKER (aka NIGHT WARNING) (1982), was also advertised to be in the picture- but wasn't. And finally, Jason Miller- who will always be remembered for his best known performance, as Father Karrass in THE EXORCIST (1973); was also due to appear- but never did. Why neither of these two vanished from the cast I don't know- any info will be added here. So get writing!