With her pixie cut, funky style, and big smile, "sex symbol" doesn't usually describe Michelle Williams. But the actress plays the part well in "My Week With Marilyn," in which she portrays one of the most famous sex symbols in history, Marilyn Monroe. Now, the 31-year-old gets her sexy on in print too, showing off her svelte bod in a bra and panties (with a cardigan draped over her shoulders) on the cover of the February issue of GQ, where she opens up about her late ex Heath Ledger, the couple's daughter, and more.

As for why she hasn't been drawn to bombshell roles before, Williams looks back to her days as a teen actress. "I think because my character on 'Dawson's Creek' was sexy…sexualized…sexual…I saw all the negative attention and connotations that can come along with that," she explains. "And that those things can keep people from seeing you clearly."

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But now might her attitude on the topic be shifting at the ripe old age of 31?

"It's funny spending your twenties running away from it, and then you hit your thirties...wait a second! Come back! I want that time back when I didn't appreciate what I had!," admits Williams. "That kind of ripe sexuality, when you hit your thirties you feel more in possession of it, you feel it's not something that can be sort of taken away from you and reassembled."

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Inside the mag are more seductive pics of a platinum-haired Williams posing in lingerie, but also some emotional comments from the actress about her daughter Matilda, and Matilda's father, Williams' ex, actor Heath Ledger, who died of an overdose four years ago.

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"Our initial meeting, the circumstances of how we first met, were cosmic or something," she says of first connecting with Ledger on the set of "Brokeback Mountain." Shortly after they began dating while still shooting, Williams became pregnant. "It's a bit like: We had a lot of things to do, because we didn't have a lot of time, or something."

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The couple's daughter, Matilda, is now 6 years old, and though Williams reveals that a year or two ago she was pressuring herself to find a new, meaningful relationship with someone she could have additional children with for Matilda's sake, she's now giving herself a break. "I really wanted, and I really expected or imagined, that Matilda would have siblings that were close to her age. I wanted that for her. But I couldn't make that happen," says the "Blue Valentine" star. "And now that she's 6 that isn't even a possibility anymore. So something that was making me feel impatient, that's been removed. For whatever reason, that's not our luck, or our path."

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Williams also refers to the guilt she feels over the fact Matilda lost her father at such a young age, without enough to remember.

"You know, as hard as certain things have been for me, it's been harder thinking about how things will be for her," Williams confesses. "I have a lot of things that she doesn't, and some of what I have I can give to her—the memories that I have, the objects that I have, the physical reminders that I have, the stories."

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And thinking about how things could have been, had Ledger lived, had the couple reconciled, not surprisingly, strikes a nerve with the actress.