The film's premise: A 16-year-old precocious London schoolgirl named Jenny meets David (Peter Sarsgaard), a man twice her age, and decides to lose her virginity to him on her 17th birthday. The subject is handled sensitively by Danish director Lone Scherfig and British screenwriter Nick Hornby.
While Polanski was not on the mind of best-selling novelist Hornby when he wrote the screenplay, none of the cast members has been able to escape the comparison.
"It came up a lot last week," Mulligan said in an interview with ABC News Now's "Popcorn With Peter Travers."
"We never thought about it. There's a line at the end of the film when [co-star] Sally Hawkins says to me. 'You're just a child' and Nick told me he now wishes he hadn't put it in."
"An Education" originated from a true story. Journalist Lynn Barber wrote an article in Granta magazine, describing her affair with a man in his 30s when she was a schoolgirl in the early 1960s. Hornby happened to read the article and the rest is history.
Mulligan plays Jenny, who is living in England a couple of years before the Beatles burst on the scene in 1963 or, as Mulligan puts it, "in the 60s before they got interesting."
Jenny is living with her parents, who do not understand her, and is headed to Oxford University after she graduates. The only problem is "she's bored," Mulligan said. "She doesn't want to be British because England is boring. Everyone is stagnant. Paris is where she belongs ... and one day she's standing in the rain and a man in a red car pulls up."
Unlike Mulligan, who, in real life, would not have gotten into the car of a total stranger -- "Now it would be crazy" -- Jenny is bolder. She is sucked into David's life, which includes whirlwind excursions to the city of her dreams, Paris.
Sarsgaard described David's motivation as a man who was not focused on having sex with a 16-year-old but rather wanted to be 16 again.
"He's not a villain," Mulligan said. "He's trying to fit in. He's never on an even keel with anyone. Peter had the idea that he didn't have a childhood, so he's just a guy who wants to be 16 again and live it for the first time."
Mulligan has won rave reviews for her portrayal of Jenny and the 24-year-old British thespian will receive the Hollywood Breakthrough Actress Award at the 13th Annual Hollywood Film Festival and Hollywood Awards Oct. 26, 2009. She holds her own opposite an impressive cast that includes Sarsgaard, Hawkins, Emma Thompson and Alfred Molina.
Mulligan's rise to fame has been sudden. Her breakout role was playing Kitty Bennet, one of Keira Knightley's little sisters, in 2005's "Pride & Prejudice." She had no prior film experience and credits British actor and Oscar-winner screenwriter ("Gosford Park") Julian Fellowes for helping her secure an audition.
Fellowes had spoken at Mulligan's school and she simply decided to write him a letter.
"He's the one who set me on the path, which led to an audition for "Pride & Prejudice," she said. "I had to audition like four times. I had to work. They were incredibly generous and a series of lucky things happened and people were generous with their time."