The 17-year-old former child star takes on a firmly grown-up role as the title character in "Effie," a movie that recreates an infamous Victorian love triangle involving 19th-century art critic John Ruskin, his disenchanted young bride Effie Gray and artist John Everett Millais.
"I think actually in films I've done more adult things in other ones than I have in this one," Fanning said during a break in filming at a country mansion near London. "But this is my first time being a wife and all of that, which is actually kind of exciting and very fun.
"I get married in the movie. I had a wedding dress. It was kind of surreal," said Fanning, who has been acting for a decade since she grabbed attention, aged 7, as the daughter of Sean Penn's character in "I Am Sam."
Fanning, who recently began studies at New York University, is no stranger to edgy material. She played a troubled 12-year-old in 2007's "Hounddog," which contained a controversial rape scene, and was the cruel vampire Jane in the "Twilight" films.
Last week Britain's advertising watchdog pulled a Marc Jacobs ad featuring Fanning, saying it seemed to sexualize a child - the regulator said the actress looked younger than 16. The ad for "Oh, Lola" perfume showed the star in a short dress sitting with an oversized perfume bottle between her legs.
Her outfits for "Effie" are considerably more demure. Written by Academy Award-winning actress Emma Thompson and starring Thompson's real-life spouse Greg Wise as Ruskin, the movie is allowing Fanning to fulfill a long-held dream of appearing in period garb.
"Wearing a corset every day has been ... interesting," she said with a laugh. "It really does make you feel like you are someone else in a different time. It takes a while to get dressed in the morning and to take everything off, and you kind of get a glimpse into what it was like not being able to take your own clothes off.
"You have to have someone helping you and putting you into these clothes. It just shows how different everything was."