Debra Tate was so supportive of Margot Robbie playing her late sister Sharon in Quentin Tarantino’s Once Upon a Time...in Hollywood that she lent the actor Sharon’s remaining, partially used bottle of perfume and some pieces of her jewelry. Five decades after the Manson family murders, Sharon’s 1969 death is still the subject of sensationalist films, books, and morbid internet lore. And Debra—recognizing the movie’s potential to acquaint mass audiences with Sharon’s lesser-known life and career—made herself fully available to Robbie as a resource.

“I wanted to do anything I could to help her feel as much like Sharon as she possibly could,” Debra told Vanity Fair Wednesday. Her sister had acted in six films in the two years before she was murdered—including The Wrecking Crew opposite Dean Martin, Twelve Plus One opposite Orson Welles, and Valley of the Dolls, for which she earned a New Star of the Year Golden Globe nomination. According to Debra, she also appeared on over 250 magazine covers worldwide. “She took her craft very seriously and was growing as an actress,” Debra said of Sharon, who died at age 26. “I feel that there are a lot of similarities between Ms. Robbie and Sharon Tate in their own rights.”

Tarantino reached out to her after he finished the script for Once Upon a Time...in Hollywood, a nostalgic homage to 1960s Tinseltown that, according to Debra, took the filmmaker five years to complete. The film chronicles three social strata of the industry—with Sharon Tate, as played by Robbie, and her husband Roman Polanski (Rafał Zawierucha), fresh off the success of Rosemary’s Baby, depicted as the most successful. Leonardo DiCaprio plays a fading television actor yearning for the kind of opportunities and cachet offered to Tate and Polanski. And Brad Pitt plays DiCaprio’s onscreen stuntman, rounding out the lowest rung of the industry—a man who devoted his life to entertainment with little to show for it. Debra said that Tarantino drove to her California town, spending three days there to let her read the script. The pair also discussed Sharon’s life and career—though, by that point, Tarantino’s screenplay was pretty much complete. Debra said that the filmmaker did, however, manage to “implement ever so slightly a few suggestions” she had.

Shortly after the movie’s Cannes Film Festival premiere in May, Tarantino was criticized for not giving Robbie—his female lead in the film—more lines. During a press conference, the filmmaker called Sharon the “heart” of the film—yet, a reporter pointed out, the character has significantly less dialogue than the male leads. Asked about this criticism, Debra said, “When I read the whole script, I knew that the real meat of the story was going to be Leo and Brad Pitt’s characters. I mean, I really wish that Quentin Tarantino would do the Sharon Tate story, and I would love to see Margot play [that]...But that was not the movie that Quentin had written, and I knew it and I understood it. And it was his vision. I’m not going to tell anybody that has done such a wonderful job, and a respectful job at honoring a particular situation, that they have to do my version of a story. Although I really wish he would’ve.”

Tarantino invited Debra to set the day that Robbie filmed a particularly moving scene as Sharon, in which the pregnant actor giddily attends a matinee screening of The Wrecking Crew. Unaccustomed to seeing her name on a marquee, she excitedly tells the ticket taker that she appears in the film. When she requests a photo, Sharon happily obliges. Debra watched Robbie film the scene from inside the theater—where Tarantino had tucked her to ensure that paparazzi did not see her on set. Because of the distance, Debra could not see Robbie clearly. But through the production headphones she was wearing, she was startled to suddenly hear her sister’s distinctive voice.