Sisters

• Sisters ran on a $500,000 budget from producer Edward R. Pressman and got distributed by American International Pictures, the low-budget exploitation shop that let De Palma keep creative control in exchange for keeping costs down.
• Margot Kidder landed the dual role of Danielle and Dominique partly because she and De Palma were involved romantically at the time, and she was already roommates in Los Angeles with co-star Jennifer Salt — a real-life closeness that fed the film's twisted-sisters dynamic.
• Bernard Herrmann, the composer behind Hitchcock's Psycho, Vertigo, and Marnie, came out of semi-retirement to score Sisters after editor Paul Hirsch temp-cut the film using Herrmann's own Hitchcock scores; De Palma liked the fit so much he tracked Herrmann down and asked him to do it for real.
• De Palma has said the film's signature split-screen murder sequence grew out of watching nightly news coverage of the Vietnam War on TV — the same split-attention, can't-look-away voyeurism the movie turns back on its audience.
• The premise traces back to a 1966 Life magazine photo essay on Masha and Dasha Krivoshlyapova, real Soviet conjoined twins — De Palma held onto the article for years before it became the seed of Danielle and Dominique.
• The film's 1973 UK cinema release was trimmed by the BBFC, which cut down the stabbing of Philip Woode before British audiences could see it uncut.

Danielle and Dominique never were truly separate people. The entire film is a dramatization of dissociative identity disorder, with the 'separation surgery' functioning as a psychological metaphor rather than a literal medical event. Grace's final asylum vision confirms she's inside Danielle's fractured consciousness, not a physical ward.

Grace Collier is the film's true protagonist of identity dissolution, not Danielle. Her investigation mirrors the process of psychoanalysis, and by the climax she's absorbed Danielle's pathology. The hypnosis scene isn't just about silencing a witness — it's about the contagious nature of trauma. Grace's blank stare in the final shot suggests she's become a vessel.

De Palma structures the entire film as a critique of spectatorship itself. The window is the camera, the courtyard is the theater, and Grace's helplessness during the split-screen murder mirrors the audience's complicity. We watch, we know, and we're ultimately hypnotized into forgetting. The asylum ending implicates everyone who's ever enjoyed watching violence on screen.

Margot Kidder plays Danielle Breton, a French-Canadian model with a conjoined twin sister she'd rather forget. She picks up ad salesman Philip Woode on a Candid Camera-style game show, brings him back to her Staten Island apartment, and by morning he's dead — stabbed by her twin, Dominique, who isn't supposed to still exist. Across the courtyard, nosy journalist Grace Collier watches the killing happen through her window and can't get the cops to believe her, so she hires private eye Joseph Larch and starts digging herself. De Palma splits the screen, splits the psyche, and splits the difference between Hitchcock worship and something genuinely unhinged. The asylum climax dissolves identity until nobody's sure who survived the separation surgery. The doctor knows. The audience won't.