While speaking with Vanity Fair for its July cover story, the actress -- who is the daughter of Melanie Griffith and Don Johnson, and Hedren's granddaughter -- revealed her family's reaction to watching the TV movie, in which Hedren was played by Sienna Miller.
And Johnson, 32, called out HBO for not giving her family a heads up before they screened the film.
"We sat at HBO, my family, and watched that movie together," she said. "It was one of those moments where you're just like, How could you not have warned us? We're in a room with some execs. Maybe this warranted a little conversation beforehand?"
"You look over and you see a woman who's just been reminded of everything she went through, and it was heartbreaking," Johnson added of Hedren, now 92. "She was an amazing actress and he stopped her from having a career."
Hedren famously worked with Hitchock on 1963's "The Birds" and 1964's "Marnie." Hedren has previously alleged several instances of abuse at the hands of the late director, who was known to have been obsessed with the actress.
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"What happened with my grandmother was horrific because Hitchcock was a tyrant," Johnson said of the filmmaker during her interview with Vanity Fair. "He was talented and prolific -- and important in terms of art -- but power can poison people."
The "Fifty Shades of Grey" star also revealed her mother received a rather disturbing gift from Hitchcock when she was just a child. Johnson said Hitchcock gifted Griffith a tiny replica of Hedren in a little coffin.
"It's alarming and dark and really, really sad for that little girl," Johnson shared. "Really scary."
"The Lost Daughter" star has opened up about her grandmother's relationship with Hitchcock in the past.
"[Alfred] Hitchcock ruined her career because she didn't want to sleep with him, and he terrorized her," she explained. "And [he] was never held accountable."
Johnson added, "It's completely unacceptable for people in a position of power to wield that power over someone in a weaker position, no matter the industry."
The "Cha Cha Real Smooth" actress then paused and appeared to become emotional as she continued. "It's hard to talk about because she's my grandmother," she said. "You don't want to imagine somebody taking advantage of your grandmother."
Hedren has previously spoken publicly about working with Hitchcock. According to The New York Post, the actress claimed in her 2016 memoir, "Tippi," that she was sexually abused by the filmmaker, who passed away in 1980. In the book, Hedren detailed the several alleged instances of abuse.