“You hire [stylist] Erin Walsh and you trust her,” Hathaway tells PEOPLE of the secret to the perfect Cannes wardrobe. “You know, I’ve never been to Cannes, and I’ve been watching the fashion of the film festival unfold for a couple decades now. And I’ve noticed certain things you can never go wrong with.”

“You never go wrong with a little sparkle. You never go wrong with something that looks chic and glamorous, but feels very, very comfortable. And I noticed that a lot of my favorite actresses had their first experience wearing all white,” continued the Academy Award winner. “So that was what I did.”

Hathaway posed solo and with her Armageddon Time cast, showing off the dress from all angles (though a source tells PEOPLE she removed the stole and train once inside the theater).But the true centerpiece of her look was her Bulgari Mediterranean Reverie necklace; a release from the brand, which also announced that Hathaway is its new ambassador, touted “the enchanting splendor of a unique stone — 107.15 dazzling carats of royal blue cushion cut sapphire from Sri Lanka.” (She paired it with a coordinating 18-carat sapphire ring from the same “Eden the Garden of Wonders” collection.)

The Princess Diaries actress’s look is the latest head-turning getup she’s been photographed in recently.

While heading to The Late Show with Stephen Colbert to promote her new series, WeCrashed, in March, Hathaway looked vibrant in a rainbow ensemble from Christopher John Rogers — the Louisiana-born designer whom celebrities can’t get enough of.

The look, which featured striking green, red, purple and orange shades, was made of a structured bustier top, palazzo pants and a coordinating blazer. Hathaway finalized her look with a turquoise La Medusa Mini Bag by Versace.

“Andy? Is that you? 😆😉,” one person commented. Someone else wrote, “Oh Andy, you look so Chic! ❤️❤️❤️❤️.”