The reason it's here is because it was produced by one of my favorite movie-hustlers, William Castle, who produced many a cheap, gimmicky horror film (so successfully that he inspired Hitchcock to make Psycho!) Hopefully, you've read this month's post about The Tingler and there are others around here somewhere—I try to post at least one Castle feature every Hallowe'en season. He'd bought the film-rights to Ira Levin's book with the intention of directing it for Paramount, until Robert Evans forebade it lest the movie look "cheap" and successfully approached Roman Polanski to make it. Castle got the producing credit and a cameo.
And Polanski made the better film. Castle would have kept the "camp" elements, certainly, but I'm not sure he would have rooted it so deeply in the real world, with references to "plain ordinary" Lipton's Tea, Vidal Sassoon hair-styles (not without some payment, at least) which is one of the most horrorific aspects to it: the movie leaves us with "the anti-Christ" and wanders off into the world, leaving us with the knowledge that "life goes on normally" even if, behind closed doors and black-out curtains, things aren't normal at all. Talk about instilling paranoia! Even Castle would have a hard time doing that, no matter how many seat-buzzers and glowing skeletons he could pack into the theater.
No, Polanski wanted to make things as normal as apple-pie, even casting familiar faces as the Satanists—my favorite is probably Hope Summers (who played Aunt Bea's neighbor, Clara, on the "Mayberry" shows and seemed to be on every sit-com known to man and Beast). And the seemingly comic busybodies who make up her neighbors are certainly no threat, although they are odd and set-in-their-ways, and Polanski's treatment of them might be chuckle-inducing...with little touches like Ruth Gordon's Mrs. Castanet smudging the place where Rosemary's knife cut the floor (don't want to ruin the finish!) or Patsy Kelly's obnoxious Laura-Louise sticking her tongue out at Rosemary as she slunks back to her pity-party (Satanists can be SO petty!) And the whole situation where the displaying of the anti-Christ resembles nothing so much as a neighborly baby-shower...or the world's worst Tupperware party.
But, he also could be giving us a warning, too. Informality doesn't necessitate complacency.
Don't get comfortable.
Oh. And ("...you haven't been, really") don't forget to vote.
The Set-up: There are difficult pregnancies and there are DIFFICULT pregnancies (emphasis on the "cult"!). From the Premiere Magazine intro: "Mia Farrow plays Rosemary Woodhouse, a waifish young housewife, who has been unwittingly impregnated by Satan. Unknown to Rosemary, her elderly neighbors, Roman and Minnie Castavet (Sidney Blackmer and Oscar-winning Best Supporting Actress Ruth Gordon) are a couple of witches who head a coven (which includes Rosemary's actor husband Guy, played by John Cassavetes; the oddball Laura-Louise, played by Patsy Kelly; and Hope Summers's Mrs. Gilmore). Here, just days after giving birth (she has been told that her baby died), she sneaks into the Castavets' apartment (they are supposed to be in Europe), kitchen knife in hand, having been drawn by activity and voices. In the living room she finds the coven and — yikes! — a black bassinet with an upside-down cross hanging over it.
Action.
Those who sit with their back to Rosemary turn around
questioningly. Guy starts to rise but sits down again.
Mr. Castevet nods and the Japanese looks at Rosemary
with interest.
JAPANESE
Ah, sssaasassssss.
Watching them, Rosemary starts across the room toward
the bassinet.
MR. CASTEVET
Rosemary.
ROSEMARY
Shut up.
MR. CASTEVET
Before you look at-
They stir and look to Mr. Castevet.
She moves from the bassinet ready to kill him.
Mr. Castevet cries, his voice growing louder and
prouder, his bearing more strong and forceful.
VOICES
Hail Satan!
Rosemary shakes her head.
ROSEMARY
No.
MRS. WEES
Hail Satan.
Laura-Louise uncovers her mouth. Guy looks out at
Rosemary from under his hand.
She covers her face.
VOICES
MR. CASTEVET
(Thundering)
God is DEAD!
ROSEMARY
Oh God! oh God! oh God!
MR. CASTEVET
God
MR. CASTEVET
Hail Satan! Hail Adrian!
Hail Adrian! Hail Satan! The year is One, God is done!
The year is One, Adrian's begun!
Rosemary backs away.
She backs further and further away through the shouting
people. In the confusion of movement, a faint fragment
of her dream flashes. A chair is behind her; she sits down on it and stares at them.
Rosemary looks at Mr. Castevet. He straightens up.
The doorbell rings.
MRS. WEES You shut up.
She comes to Rosemary and puts a dampened handkerchief
in her hand.
MRS. WEES
Rosemary is His mother, so you
show some respect.
Laura-Louise mutters.
Rosemary wipes her forehead and cheeks with the hand-kerchief. The Japanese, sitting across the room on a
hassock, catches Rosemary's eye, grins and ducks his
head. He holds up an opened camera into which he is
putting film. Rosemary looks down and starts crying.
Mr. Castevet comes in, holding the arm of ARGYRON STAVROPOULOS. He is a robust, handsome, dark-skinned man, wearing a white suit, white shoes and carrying a large box wrapped in light blue paper patterned with Teddy bears and candy canes. Musical sounds come from it. Everyone gathers to meet him and shake his hand. There is a confused, hushed conversation from which words like "Worried - pleasure -- airport - Stavropoulos -- occasion" can be heard.
Laura-Louise
brings the box to the bassinet. She holds it up for
the baby to see, shakes it, and puts it on the
window seat. There are other boxes similarly wrapped
and a few that are wrapped in black with black ribbon.
Mr. Castevet draws Argyron Stavropoulos forward. 
MR. CASTEVET
Come, my friend. Come see Him.
Argyron Stavropoulos lowers himself
to his knees.
Mr. Castevet catches Guy by the arm.
He looks over Guy's shoulder, at Rosemary.
Mr. Castevet draws him away, speaking in his ear.
Rosemary looks at the mug and looks up at Mrs.
Castevet.
MR. CASTEVET
Let Rosemary rock him.
MR. CASTEVET
Sit down with the others, Laura-Louise.
He smiles at Rosemary and moves the bassinet back and
forth towards her, holding it by its hood. Rosemary
stands still and looks at him.!
ROSEMARY
You're trying to get me to be his
mother.
-
For
a few moments they rock the bassinet between them,
then Mr. Castevet lets go and Rosemary rocks it alone,
nice and slowly.
Mr. Castevet withdraws silently to
where everybody now stands in a semi-circle, watching.
Dr. Sapirstein comes into the room and looks at the
scene, in surprise. 
Mrs. Castevet puts her finger to
her lips. 
The Japanese steps forward
Words by Roman Polanski (and Ira Levin)
Rosemary's Baby is available on DVD, Blu-Ray and 4K UHD from Paramount Home Entertainment and The Criterion Collection.
* Mad Magazine's parody of the film had Rosemary give a great rejoinder: "Blood-red eyes? Who's his father? Dean Martin?" Oh. And for those of you who notice such things (Being raised Catholic, I did!), the capitalization of "His" and the entity's name are from the script (Being raised Catholic, I had to fight an impulse to change it...once you're in "the club", you know). They do it in religious texts for Supreme Beings in the religions that I know, anyway, so...)