Rosemary's Baby is perhaps the single most faithful adaptation of a book to film ever created – with a script that's less an adaptation than a near-literal transcription of Levin's novel, carrying over its most minute details: its 'slow-burn' tension-building, its ratcheting paranoia, its scene structure, dialogue, descriptions, its dark wit, its is-it-real-or-is-it-imagined ambiguity.
In fact, you'd be hard-pressed to find any material elements in the film that aren't direct transfers from the book:
“[T]he film has not a single idea that does not derive from the book (well, maybe one or two – but they are borrowed from Hitchcock). In fact the film is so full of the book that viewers acquainted with the original may feel cheated when some of Levin’s ideas fail to turn up. Not that Polanski’s work is inferior — it just isn’t original.”—Christian Century (1968)
Even many of the movie's signature visuals hail from the book: Minnie Castevet through the peephole... Guy and Roman seated just out of view... the entire six-minute 'dream' sequence... They're all stroke-for-stroke renderings of the novel's descriptions. Even much of the costuming comes straight from the book. (Ditto the Vidal Sassoon haircut.)
That's no slight to the film – its direction, performances, cinematography, production design, and brilliant Krzysztof Komeda score combine with its very unflinching fidelity to Levin's novel to create a classic that now deservedly resides in the National Film Registry.
So it comes as no surprise that Levin found Rosemary's Baby to be the most successful film adaptation of any of his works, by a wide margin. Living at the time of its release in Connecticut, he told the Westport Town Crier's Barbara Plunkett “I couldn't ask for a more faithful job,” though Plunkett herself found the film to be “lacking some of the high suspense and fright of the novel.”
Indeed, as faithful as the film is, it still omits significant additional layers present in the book.
According to the New York Post of September 30, 1967 – the Dakota's exterior was rented out for $1,000 a day — with one of the co-op's owners stating "The only kick is we didn't make more. It came to $4,000 or $5,000."