Lily James is Cinderella in Disney's live-action feature inspired by the classic fairy tale, CINDERELLA. (Jonathan Olley/Courtesy of Walt Disney Studios)

A natural brunette, Lily James was happy to dye her hair blond when she landed the role of Lady Rose MacClare on the hit drama “Downton Abbey” — she just had no idea how much her makeover would change her life.

When James auditioned to play one of the stepsisters for Disney’s upcoming big-screen adaptation of “Cinderella,” the casting director noticed her new hair color and offered her a suggestion: “You should just read for Cinderella while you’re here.”

Surprised, James read a few lines, figuring her mostly unknown name had no shot at such a coveted leading role. Her agent advised her to have fun with it, just for the experience. Can you guess what happened next?

If you don’t know, then you’re not one of the millions who have watched the YouTube trailers for “Cinderella” (opening in March) starring James, 25, as one of the most famous fairy-tale figures of all time. Cate Blanchett plays the Evil Stepmother, and Helena Bonham Carter is the Fairy Godmother.

James, a British actress who began her career in theater, is feeling the pressure of updating Cinderella sensibilities for yet another version of the famous story — she spent hours talking with director Kenneth Branagh about the character.

There also were more practical challenges: using CGI for the glass slipper (“No maiden in the land fits into that shoe”) and Cinderella’s signature sweeping blue gown.

“The dress that [costume designer] Sandy Powell created — I mean, I think she’s a genius and I’m grateful for that dress — but it was like torture,” James joked, estimating it took about 45 minutes to get in and out of the 12-layer garment. “It was so tight and delicate . . . so it did feel quite magical every time I put it on. It did all the work for you.”

By coincidence, James’s next upcoming starring role is also playing an iconic character — with a twist. She stars as Elizabeth Bennet in “Pride and Prejudice and Zombies,” based on the novel that infused sci-fi into the classic Jane Austen love story.

James laughs as she remembers how she said “absolutely not” when she first heard about the part and how quickly it turned into “absolutely yes” after she read the book. Turns out, it’s a lot of fun getting to play one of literature’s most famous heroines while simultaneously learning martial arts and chopping off zombie heads.

She recalls filming one of her favorite scenes, just because of the absurd juxtaposition: She is seated with co-stars Suki Waterhouse and Bella Heathcote, who play Elizabeth’s sisters.

“They’re these brilliant, beautiful girls,” James recalls. “And instead of sitting around knitting, we were sitting around polishing muskets.”

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