The David Fincher movie is based on the life of Hollywood screenwriter Herman J. Mankiewicz and chronicles the tumultuous development of Orson Welles' iconic 1941 film "Citizen Kane", with Gary Oldman starring as the titular character.
Seyfried portrays Davis, alongside Charles Dance as her lover, media magnate William Randolph Hearst, but the "Mamma Mia!" star admits finding a way to tackle her role proved to be a little tricky at first - until she stepped onto the 1930s era set.
"Fincher doesn't make any mistakes, really. He creates a world that feels so authentic, and when you think about 1930s Hollywood, it just feels so far away, and I always saw myself as a very modern actress," Seyfried shared on U.S. breakfast show "Today".
"I was like, 'How am I gonna jump into this?' But it was really kind of easy in some ways because I felt like the costumes, the makeup, the hair, the set pieces, it was all there."
"It was a dream that I didn't know I had. How many opportunities do you get to play a 1930s era movie star... like this?"
As part of her portrayal of Davis, the Pennsylvania-born Seyfried had to master a Brooklyn, New York accent.
"My trick is do everything very subtly," she explained. "Mild is really the only thing I think I can get away with (with) something that's not natural; it's not a natural accent for me."
"My biggest fear is to distract people and (for) it (to) sound terrible, so I just did a mild Brooklyn accent!"
Her efforts appear to be paying off as Seyfried's performance in the Netflix movie has already earned her high praise from critics, and the actress is considered to be one of the early favourites to land an Oscar nomination for the 2021 Academy Awards.