Michelle Williams, a perennial fan of floral dresses and peplums, usually sticks to a pretty tame style palette.

But all Peter Pan collars were off at Thursday night's "Take This Waltz" screening, where Michelle ditched her girlie duds (at least until Friday morning) for a seriously sexed-up look.

In a sexy lace-up Altuzarra dress and strappy white Givenchy heels, Williams bared a hint of cleavage as she promoted her new movie alongside co-star Luke Kirby. In the movie, Williams and Seth Rogen play a married couple whose relationship is tested by an attractive new neighbor. (Kirby, obvi -- look at him!) Boyfriend Jason Segel, unfortunately, was nowhere in sight.

Speaking of attractive folks in Hollywood... Michelle's not looking too bad herself.

Check out Williams' new look below!

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