Before the filming of the scene of Rosemary calling Donald Baumgart (the actor in the story who mysteriously goes blind), Mia Farrow did not know who would be speaking the lines. It was Tony Curtis, and in the scene Farrow shows slight confusion, finding the voice familiar but not able to place it. This confusion was exactly the effect director Roman Polanski hoped to capture by having Curtis read the lines.
Directed by Roman Polanski, whose pregnant wife actress Sharon Tate was murdered in 1969 by Charles Manson and his followers, who titled their death spree "Helter Skelter" after the 1968 song by The Beatles, one of whose members, John Lennon, would one day live (and in 1980 be murdered) in the Manhattan apartment building called The Dakota - where Rosemary's Baby had been filmed.
William Castle acquired the movie rights to the novel. Robert Evans of Paramount agreed to green-light the project if Castle did not direct. This was due to Castle's fame and reputation as a director of low budget horror films. Castle was allowed to make a prominent cameo appearance.
According to John Parker's recent biography of Jack Nicholson, Robert Evans suggested Nicholson to Polanski but, after their meeting, the director stated that "for all his talent, his slightly sinister appearance ruled him out".
Roman Polanski was so faithful to the novel that he asked Ira Levin the date of the issue of the New Yorker in which Guy Woodhouse sees a shirt he wants. Levin confessed that he had made up the detail.
Rosemary (Mia Farrow) says to Terry Gionoffrio (Angela Dorian), "I thought you were Victoria Vetri, the actress," to which Terry responds, "Everyone says that, but I don't see the resemblance." Victoria Vetri is Angela Dorian's real name.
A scene was shot, but not used, of the characters attending an off-Broadway play. Mia Farrow's and Emmaline Henry's attend a performance of "The Fantasticks" and meet Joan Crawford and Van Johnson as themselves. Along with several other insignificant scenes, this was deleted to reduce the film's running time.
The script called for Rosemary (Mia Farrow) to explain to Guy (John Cassavetes), that she'd "been to Vidal Sassoon" for her dramatic new haircut. Thus, Vidal Sassoon was in fact flown to the set to arrange Mia Farrow's hair into the now iconic pixie cut she sports during the second half of the film. For the first part, she wears a blonde wig designed by famed stylist Sydney Guilaroff.
The book that Rosemary reads in the cab is the Book of Ceremonial Magic, by A.E. Waite, Chapter IV: The Rituals Of Black Magic: Section 4: The Grimoire of Honorius. The italic section has been entered into the natural flow of the text; the previous paragraph has been shortened to make space for it.
When Rosemary receives the book "All of Them Witches," she is told that 'the name is an anagram.' At first she tries to rearrange the letters of the book's title, but then realizes that the clue referred to a name within the book. However, the title actually is an anagram for 'Hell a Cometh Swift.'
In a scene where Rosemary is getting her blood drawn, Rosemary tells the doctor that she just saw the off- Broadway show "The Fantasticks." In that play, the parental figures arrange a "rape" of the ingénue (meaning, in this case, "abduction," from the Latin "rapere," rather than sexual assault), by a dark devilish character (named El Gallo), so a young man can save her, hoping that the young girl fall in love with the young man and marry him.
Ira Levin felt Rosemary's Baby is "the single most faithful adaptation of a novel ever to come out of Hollywood." William Castle speculated the reasons for this were because it was the first time Roman Polanski had ever adapted another writer's work. Unaware he had the freedom to improvise on the book.
Rosemary's Baby is almost an exact adaptation of the novel on which it's based. Most of the material left out during adaptation was done so for time, but the fast majority of all events and characters in the film play out the same way and in the same order as they do in the book.
The book Hutch leaves for Rosemary has a passage referring to Tanis root, the substance in her necklace. The full paragraph reads, "In their rituals they often use the fungus called Devil's Pepper. This is a spongy matter derived from swampy regions having a strong pungent odor. Devil's Pepper is considered to have special powers. It has been used in rituals and worn on charms."
Many scenes are shot in a continuous unbroken take or with minimal cuts in an unnoticeable way such as the opening scene where Rosemary and Guy first tour their apartment (two cuts), the laundry room scene (only one cut), the "let's have a baby" scene, the New Year's Eve party, Rosemary's and Guy's argument after their party, Rosemary's getting the unfortunate phone call about Hutch, the final scene at Dr. Sapirstein's office where she tells him of Adrian Marcoto, Rosemary's phone call with Baumgard, and the famous phone booth scene.
The trivia items below may give away important plot points.
Mia Farrow went on to play the nanny to "devil child" Damien in the 2006 remake of The Omen almost 40 years later. The film was released to coincide with the date 06/06/06 in order to spark interest and gain publicity.
In the novel, Minnie deduces that Rosemary has found out about Roman's father because of Rosemary's sudden behavioral distance. Minnie confronts Rosemary about it, condemning Adrian Marcoto, saying, "I could kill that crazy old man if he wasn't dead already; he's been the bane of poor Roman's existence!" and further explains that that's the reason for the Castevets' persistent traveling because they can't stay in one place for too long without people finding out.
In the novel it explains that the raw meat Rosemary eats after the New Years Eve party is a chicken's heart. In the olden days, chickens and hens were used to aid witch hunters and were believed to be symbols of God and light. So, because of the unholy child inside her, this explains why the chicken's heart was the only raw meat that made Rosemary sick.
Producer William Castle wanted to display a grotesque demon baby at the end of the film when Mia Farrow looks at her child but Roman Polanski (and the other producers) vetoed the idea in lieu of a more ambiguous scene.