Lily James can't believe the furore over the size of her waist in the new cinematic retelling of Disney's Cinderella.

"I feel like I've got to go, 'I'm sorry I've got a small waist'. It's a shame, because it's not what the film is about," says the 25-year-old who found fame as Lady Rose in Downton Abbey.

Tall and naturally slim, James' exercise regime differs from job to job. "With Cinderella, I did loads of yoga, because I wanted to feel graceful, and it was also to do with meditation and feeling centred," explains the actress, who says she tries to eat healthily but isn't averse to the odd tipple - even when on a health-kick. "Basically I stop drinking beer and start drinking vodka!" she jokes.

As for the critics, she challenges them to see the movie with their kids - "and see what they come out saying".

Arriving 65 years after the original Disney animation of the famous fairy tale was released, the movie, directed by Kenneth Branagh, is beautifully shot, with an adaptation fit for modern audiences. For one, the man is the love interest rather than the woman, something James acknowledges with a wide smile.

"Cinderella is all about the woman, and it's great to follow a female protagonist," remarks the Surrey-born actress, looking chic with her poker straight hair and fitted white dress. "It's not about a girl looking for a prince, if that's what you want to do, then each to their own, but with Cinderella, she's fine on her own, dealing with life and doing it really well, despite her circumstances."

In this telling of the timeless story, Ella (James) is a beautiful young woman whose idyllic life comes crashing down when her merchant father (Ben Chaplin) remarries following the death of her mother (Hayley Atwell ). She welcomes her stepmother (Cate Blanchett, on magnificent form) and her two daughters into the family home, but when her father dies, she finds herself at the mercy of her jealous and cruel new family.

"Cate's so wonderful and brings so much to the character," notes James of her Oscar-winning co-star. "When you're acting with her, you're enriched by what she's doing."

She "loves" Blanchett's interpretation of the stepmother too. "I felt such empathy for the character. I could see the pain in her, in her life and the bitterness," she adds.

One day, Ella escapes to the woods on horseback ("I was definitely out of control and going way faster than I could manage as a rider") and comes across a dashing stranger (Game Of Thrones' Richard Madden), who tells her he's called Kit and merely an apprentice at the palace.

"When she meets the prince, they meet as equals and don't know who each other are, and I think you get the sense they're rescuing each other, which I think is a better message for young girls."

An invitation for all maidens to attend a ball is sent out and Ella believes it's her opportunity to reunite with Kit - but her plans are dashed when her stepmother spitefully ruins her dress. Step forward Helena Bonham Carter's Fairy Godmother, along with a pumpkin, spectacular gown and pair of glass slippers, and you've got yourselves a happily ever after.

"I was so moved by Ella's courage and so relieved that it's about this beauty radiating from within, and that just because you're nice, or the good and kind one, doesn't mean you're a pushover or soft. You can still have guts," notes James, who believes it's the notion of rooting for the underdog that explains the story's enduring appeal.

The actress has spent the last few weeks touring the world promoting the movie, and can empathise with Ella's overwhelming predicament before she enters the ballroom.

"In my hotel room before a red carpet or premiere, I get so nervous, because it's 'you' [rather than a character]," she explains. "The other night [at the UK premiere], my earring broke just as I was about to leave. So I ended up wearing odd earrings and thought, 'I'm just going to roll with it'. So that moment where she says, 'I'm just a girl', I clung onto for dear life, because that's how I feel, how anyone would feel if they're flung into playing Cinderella."

It's only five years since James graduated from London's Guildhall School Of Music And Drama, in which time she's starred opposite Dominic West in a production of Othello and Liam Neeson in the fantasy blockbuster Wrath Of The Titans.

"It does feel like everything has happened quite fast. I've just been working so much. You go, 'Oh God, am I really here? Is that actually me on that poster?'" she says, turning to the promotional board bearing her face behind her.

"I think Downton Abbey [which she joined in 2012 and was last seen in last year] has been amazing for me, because it meant I was seen for something like this and put in rooms with people I want to work with. I'm so grateful."

Will Lady Rose ever return to the series?

"I'll be back..." James says with a grin.

In the meantime, she's busy filming a TV adaptation of Tolstoy's 1869 novel War And Peace with Happy Valley's James Norton in Lithuania.

"I've been going back and forth, which has been a really weird sensation. I feel a bit schizophrenic but it's going really well, and Natasha's such an interesting character, such darkness."

Later this year, she'll also be promoting Pride And Prejudice And Zombies, a horror parody of Jane Austen's classic, in which she stars as the book's heroine Elizabeth Bennet (James met her boyfriend, former Doctor Who Matt Smith, on set).

Beyond that, she admits she's looking for a contemporary role after all the period projects.

"I just want to feel really challenged, and push myself, so I think the best way of doing that is by doing something completely different. I'm going to say to my agent, 'Jeans, no bra, T-shirt and a cigarette, much more like me'."

:: Cinderella is released on Friday, March 27