EXCLUSIVE: Anne Hathaway Stuns in Back-Baring Gown, Gives Advice to Expectant Mothers

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Earlier this week, Anne Hathaway was showing off the $20 dress she purchased from a flea market, but on Tuesday night the Oscar winner donned a stunning 2006 Archive Armani gown on the red carpet.

“That's the Cinderella part,” she explained to ET on the red carpet of the NYC premiere of Colossal on Tuesday. “I was so excited to wear that $20 flea market dress today because I feel like that's more true to who I am. That's more true to who other people are but this, getting Armani to open up their archive, I feel like Cinderella right now.”

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Hathaway looked gorgeous in the vintage black gown that had an open back and a dramatic raised ruffle.

“Sometimes you are on these things and you feel a little shy, and I feel that I can duck for a second and gather myself,” Hathaway told ET of the bold embellishment.

In Colossal, Hathaway is tackling a role unlike anything she’s ever tried before. She plays Gloria, a party girl who awakens a giant monster in South Korea every time she gets drunk. Hathaway co-stars alongside Jason Sudeikis.

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“He's amazing. I've known Jason since 2008 when I hosted Saturday Night Live, and he was a dream to work with back then and he still is,” she said. “It’s just been wonderful to watch him come more and more into himself. He gives really astonishing performance in this. I think people are going to be really surprised.”

In addition to getting back into her work, Hathaway also just celebrated the first birthday of her son, Jonathan.

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“I can’t believe it,” she said. “Everybody -- if you're pregnant right now -- everybody is going to tell you, ‘Oh it goes so fast,’ and it’s not going to mean anything. It goes so fast like strapped a Go Pro on your head. You will be happy you did.”

For more from Hathaway, watch the clip below!