Vulture reports that Robbie was offered the role after writing Tarantino a fan-letter following filming of I, Tonya.
"I wrote him and said, 'I adore your films, and I would love to work with you in some capacity. Or any capacity,' " the actress said.
Apparently, the writer/director has already been considering Robbie for the part, and her letter came as a sign that he should have her read the script.
Robbie went to Tarantino's home to check out the screenplay, which she says took her about 4 hours to fully read. During that time, Robbie confessed that "Tarantino would occasionally pop in to offer her food or a Victoria Bitter, an Australian beer."
Tarantino wrote and directed Once Upon a Time in Hollywood, a film that revolves around the Manson Family-era of the '60s in Hollywood.
He also described the film as "a story that takes place in Los Angeles in 1969, at the height of hippy Hollywood. The two lead characters are Rick Dalton, former star of a Western TV series, and his longtime stunt double Cliff Booth. Both are struggling to make it in a Hollywood they don't recognize anymore. But Rick has a very famous next-door neighbor … Sharon Tate."
"I've been working on this script for five years, as well as living in Los Angeles County most of my life, including in 1969, when I was seven years old," Tarantino added, in a previous statement to The Hollywood Reporter. "I'm very excited to tell this story of an L.A. and a Hollywood that don't exist anymore. And I couldn't be happier about the dynamic teaming of DiCaprio & Pitt as Rick & Cliff."
In addition to Robbie, the star-studded cast of the film also features Leonardo DiCaprio, Brad Pitt, Al Pacino, Emile Hirsch, Damian Lewis, Bruce Dern, Dakota Fanning, Scoot McNairy, Lena Dunham, Clifton Collins Jr., Rumer Willis, Dreama Walker, Victoria Pedretti, Zoë Bell, Tim Roth, Michael Madsen, Kurt Russell, Timothy Olyphant, and James Marsden.
Former 90210 star Luke Perry also has a role in the film. Perry tragically passed away earlier this year after suffering a stroke. His part in the movie will be his final film role.
Additionally, legendary actor Burt Reynolds was originally set to film a part for the movie, but after his death in September 2018 Dern was brought in to fill that role.