Lily James says criticism about her waist being made to appear too thin in Cinderella is "irrelevant".
The Disney remake has faced a backlash from US parents who claim the princess has an unnaturally small waist.
The Downton Abbey actress, 25, reportedly had to go on a liquid diet to fit into the tight-fitting corset for the role.
"Why on earth are we focusing on something so irrelevant?" she asked reporters at the UK premiere.
"I've had friends' kids and a little boy Daniel who is nine and he said, 'It's amazing how you promised your mum to be kind and good and remembered it'.
"You just thought, 'That's the message.'"
The live action adaptation has been criticised ever since a trailer showed 25-year-old Lily James in a blue princess ball gown with a tiny waist.
Some people accused filmmakers of digitally resizing it to make it look smaller, but the stars denied that at the premiere.
"The film is about courage, kindness, strength and beauty from within so they're focussing on the wrong thing," James told Newsbeat.
"I had Sandy Powell create it. The first time I put it on it took my breath away.
"It's so beautiful and to wear it, I felt empowered. It felt like it was a suit of armour. I felt unbelievable."
Sandy Powell - who's won Oscars for her designs in Shakespeare in Love, The Aviator and The Young Victoria - also defended the costume.
"I think it's a really boring thing to talk about when there's an entire film," she told Newsbeat.
"The only CGI involved in the costumes is to do with the shoe.
"We made the show, and the shoe is a real object but then obviously Lily doesn't wear the shoe and the shoe is magically put onto her feet."
Other critics say Cinderella is an unhealthy female role model and that it's undone some of the work of Frozen because it suggests she needs Prince Charming to get by.
Game of Thrones actor Richard Madden, who plays the role, says that's nonsense.
"Cinderella does not need Prince Charming. She's be fine without him," he told Newsbeat.
"My job was to create a prince that was worthy of her affection.
"She would be fine without him. That's the character. She's strong and brave and doesn't need him.
"Actually they bring out the best in each other and that's why I wanted to do this because it wasn't that slightly old-fashioned message and probably not the best message to tell children, that you need a man with a big house and lots of money to rescue you from your terrible life.
"Actually you need somebody that brings out the best in you and encourages you."
Helena Bonham Carter says she was worried that Kenneth Branagh's retelling of Cinderella was going to be a "boring cliche" because there was no twist to the classic fairytale.
The Bafta-winning actress plays the Fairy Godmother.
Bonham Carter said: "I was really scared that it was going to be such a cliche and really boring.
"I said, 'What there's no take on it? No twist?'
"Cinderella is a story that's been told 100 times and everyone knows what's going to happen.
"But having seen it I'm amazed and it does work, which is testament to, one - the story is such a classic and was meant to be retold. And also he's very clever Ken [Branagh].
"It could have failed on so many levels and been really naff. But I think all the characters work because they're not stereotypes."
The film also stars Cate Blanchett as Lady Tremaine, Stellan Skarsgard as the grand duke and Derek Jacobi as the king.
Cinderella opens in UK cinemas next Friday.