- Acting for Two: Margot Kidder as twins Danielle and Dominique.
- Actor-Shared Background: Like Danielle, Margot Kidder was born in Canada in 1948 and went to New York to study acting.
- Cast the Expert: For added Mind Screw in Grace's hallucination scene, Brian De Palma brought in a bunch of sideshow performers as extras, including Eddie Carmel, the "Jewish Giant" featured in one of the most famous Diane Arbus photographs (and he also played the "closet monster" in The Brain That Wouldn't Die).
- Completely Different Title:
- Argentina: Devilish Sisters
- Brazil: Diabolical Sisters
- France: Blood Sisters
- Finland: Bloody Knife
- Germany: The Sisters of Evil
- Japan: Devil's Sister
- Mexico: Devilish Twins
- Norway: Play with Death
- Sweden: The Devil's Bloodline
- Creator Couple: Brian De Palma and Margot Kidder were dating at the time. De Palma had earlier dated Jennifer Salt, and Kidder and Salt were roommates during that era, which has led to speculation that the twisted triangle at the heart of the film might've been considered at least partly autobiographical by all three parties.
- Director Trademark: Sisters established many of Brian De Palma's directoral hallmarks; including the The Oner and Split Screen, dark psychological and sexual themes, and homages to the works of Alfred Hitchcock and Mario Bava. Another mini-thread in his career established here was the use of a sweet, vulnerable dark-haired female lead as his counterpart to the "Hitchcock blonde". Jessica Harper in Phantom of the Paradise and Geneviève Bujold in Obsession followed the precedent set by Margot Kidder.
- Fake Nationality: American character actor William Finley as the French-Canadian Dr. Emil Breton.
- I Knew It!: It's not hard to figure out that Dominique is actually Danielle's Split Personality, since the character is depicted in an ambiguous and slightly confusing way to begin with. But you really have to pay close attention to catch on that they were separated Conjoined Twins and Emil Breton was the doctor who performed the surgery before The Reveal.
- No Celebrities Were Harmed: The Peeping Tom game show is Candid Camera in all but name.
- Production Posse: The film features De Palma regulars William Finley and Charles Durning, composer Bernard Herrmann and editor Paul Hirsch.
- Real-Life Relative: Grace and Mrs. Collier are played by actual daughter-and-mother team Jennifer Salt and Mary Davenport.
- Spoiler Title: The film's title in Mexico and Argentina: Las siamesas diabólicas ("the diabolical Siamese twins").
- Very Loosely Based on a True Story: De Palma was inspired to write the film after reading an article about a pair of conjoined sisters in the Soviet Union who, upon being separated via an experimental surgical procedure, began suffering from severe psychological issues.
- What Could Have Been:
- Producer Ray Stark originally wanted the film as a vehicle for Raquel Welch.
- Danielle/Dominique was originally meant to be Swedish, but Margot Kidder was unable to convincingly pull off the accent. De Palma simply wanted her character to be non-specifically "foreign," so Kidder performed the role with a Quebecois accent and the character was changed likewise.
- De Palma shot an elaborate 6-minute long tracking shot that was ultimately cut from the film. He wrote "a Max Ophüls-type tracking shot about 6 minutes long, and while they are searching through the apartment the camera keeps coming back to the couch and the spot is getting bigger and bigger and bigger," De Palma stated. "I shot it, but because the camera could only get down so low and still go up high enough to shoot the rest of the scene we couldn't get down to the bottom of the couch and when we saw the rushes it looked ridiculous because it looked like the guy was bleeding up through the arm of the couch. So I had to throw out the whole tracking shot, and I was forced to use close-ups and television-type coverage."
- Phillip was written with Sidney Poitier in mind, but De Palma knew he was way out of the film's budget range.
Trivia / Sisters (1973)