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."
Carey Mulligan Shoots to Stardom
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."
Mulligan is dating "Transformers" actor Shia LaBeouf, who is also her co-star in Oliver Stone's upcoming "Wall Street 2: Money Never Sleeps."
Paparazzi recently captured the young on-and-off-screen couple astride a motorcycle in New York City as they filmed the opening scene of the movie. It also stars Michael Douglas, who is reprising his role as Master of the Universe Gordon Gekko.
Douglas won an Academy Award for the first picture, whose mantra was greed is good, giving his most memorable declaration as "lunch is for wimps!"
Thanks to her performance in "An Education," Mulligan was cast in "Wall Street 2."
"For the first time in my life, I didn't have to audition," he said. "He saw the film and my agent called to say that Oliver Stone is going to call me.
"'Wall Street' and 'Never Let Me Go' came as a result of the film."
"Never Let Me Go," the upcoming film based on the award-winning novel by Kazuo Ishiguro, is directed by Mark Romanek and also stars Keira Knightley and Andrew Garfield. The relative ease with which she got her latest roles is in sharp contrast with how long Mulligan waited to be cast in "An Education."
"It took almost two years," she said. "I would ring up my agent daily but there's nothing they can do. ... When I got it, I was ecstatic."
Carey Mulligan Still Rides the Bus
While Mulligan is becoming more famous for her on-screen roles, she also shined on stage as doomed, would-be actress Nina in the 2007 revival of Anton Chekhov's "The Seagull."
That was where she met Sarsgaard, who played Trigorin, the lover of Arkadina, who was brilliantly portrayed by Kristin Scott Thomas.
Sarsgaard, because he did not want her to be disappointed, had warned Mulligan that "An Education" might not be picked up at the Sundance Film Festival. But it won the audience award at Sundance and was accepted at the Berlin, Telluride and Toronto film festivals.
"Sundance has been brilliant," she said. "I'd never been to a film festival and never starred in the lead."
Her newfound fame has not gone to her head. On a recent trip to Los Angeles, Mulligan -- unlike most of her peers -- started taking the bus in Los Angeles. "I can't drive and, so, I just thought it was the easiest way to get around," she said. "Cabs are expensive, so for $5 a day, you can ride the bus the whole day. People think it's dangerous but not at all. But I wouldn't go at midnight."
One of her meetings was with actor Warren Beatty, who refused to let her take the bus and, instead, drove her to her next appointment.
"Warren Beatty is a gentleman," she said, declining to disclose what project Beatty was working on while adding that she "wouldn't say no" to his offer.
Mulligan uses music to tap into her characters and, for Jenny, she mainly listened to Edith Piaf and Juliette Gréco. She conceded that her cast members made it easier for her to connect. "In this film, most of the drama was easier for me to get into," she said. "If you do a scene with Emma Thompson, you have to be on your game."
Indeed, the movie that most influenced Mulligan before she got into film was Emma Thompson's "Sense and Sensibility." Thompson wrote the screenplay and starred in 1995 alongside Kate Winslet and Hugh Grant.
In "An Education," Jenny gets her dream and goes to Paris. For Mulligan, appearing on stage on Broadway and "having light bulbs around my mirror" was her fairytale. Although she was born in London, New York is where she is most at home.
"Last year, when I did 'The Seagull,' I cried all the way to the airport, not only because I was sad to finish the play but because I was leaving New York. I do love it here."
"An Education" opens in theaters today.