After getting into her make up for the first time, Gwyneth Paltrow spent the rest of the day walking around in public. No one recognized her, and she used people's reaction to her as an obese woman to help understand her character in the film.
When Rosemary jumps in the pool, the huge splash was created by two wide hoses (which Gwyneth Paltrow had to dive between) which shot off the streams of water on either side of her, simulating a huge splash of water.
Gwyneth Paltrow's make-up that she wears as Rosemary's true self (i.e. in a fat suit) took four hours to apply.
Sage Bonnie Humphrey: Anthony Robbins' wife gets his autograph when he is coming out of the building and Jason Alexander comes to talk to him.
Lyndon Byers: former Boston Bruins player and Boston radio personality, as one of the hecklers outside the restaurant.