The 37-year-old actress turned around to reveal the strappy back of her form-fitting gown on the red carpet, and we couldn't believe our eyes. She wore gold hoops and her hair in a swept-back low bun.
Her on-screen sister Lady Edith, aka actress Laura Carmichael, also opted for a black gown. She wore a sequin-detailed Oscar de la Renta mini dress with side train, and revealed new details about the film.
"You have a bit more time," Carmichael said of making a movie rather than a TV show. "We're playing these guys we know so well, but everything around you—the camerawork and the costumes and everything about [the movie]—has a bit more space, so we have these incredible overhead helicopter shots of the castle, and soooo many beautiful costumes."