Any sane actress would be apprehensive about playing Marilyn Monroe, as Michelle Williams does in “My Week With Marilyn.” The icon’s shoes are awfully hard to step into — and filling other garments is even more intimidating.

“I miss those hips,” Williams admits wistfully, referring to the padding that convincingly gave the waifish actress Monroe’s famous curves. She first tried gaining weight for the part, De Niro-style, but found the pounds went more to her face than her hips. It took low-tech movie magic, not Method acting, to fill out those glittering evening gowns.

Sitting in a suite at the Waldorf Astoria, a cloud of Old Hollywood glamour lingers around the actress, even if, wearing a simple dress and boyish haircut, she herself has returned to Earth. (How old-school is the Waldorf? On the 39th floor, the windows still open.) Hotel staffers have left a photo for her, of Monroe at a 1957 gala in a Waldorf ballroom — it’s one more reminder that, unlike previous roles she was able to create in private, this one belongs to the world.

“The other characters I’ve played, they exist in my imagination,” she muses. “There are ways to keep a connection with them, to never really have to say goodbye totally, but the world doesn’t offer them up as often.”

One assumes that’s even true of a role Williams played for six years: Jen Lindley, the “Dawson’s Creek” character who made her famous. Williams has done a remarkably good job of segueing from TV teen drama to being seen as one of her generation’s most serious film actresses — earning Oscar nominations for “Brokeback Mountain” and “Blue Valentine” while forming relationships with indie auteurs like “Wendy and Lucy” director Kelly Reichardt.

But unanimous critical respect didn’t keep her from suffering deep self-doubt on the set of this film, which chronicles the making of the Monroe/Laurence Olivier comedy “The Prince and the Showgirl.” (Kenneth Branagh plays Olivier.) Whether out of worry over her own performance or in subconscious emulation of Monroe — who suffered crippling insecurities about her acting — Williams says she continuously felt intimidated and “more dependent on people than I have in the past.”

“I lived on people’s compliments, kind words,” she says. “An approving glance would last me days.”

When complimented on the way she brought a couple of Monroe’s famous song-and-dance numbers to life, she responds with delight and convincing humility. Though her earliest experience as a child actor was in musical theater, Williams says flatly that she’s “not a singer or a dancer.”

If the musical sequences were “pretty daunting at first,” though, they also offered a temporary respite from self-doubt. “When you’re singing and dancing, your head turns off. Your critical brain is not allowed any room. Because you’re, you know, you’re basically trying to do this” — she pats her head and rubs her stomach simultaneously.

There was only one moment at which the actress was really able to accept the illusion of herself as Monroe. Photographer Brigitte Lacombe was on the set (at Pinewood Studios near London) and visited Williams in her dressing room — the same dressing room Monroe used while shooting “Prince and the Showgirl.”

“She came and documented the entire [makeup] process, from me sitting down in the morning, going through the three-hour transformation, and then as Marilyn — which was one of Marilyn’s very favorite things to do, to have her picture taken, because she got such confirmation of her existence and her beauty. And when I saw those photos, I thought” — she whispers now, as if marveling at a rare creature — “Oh, there she is! There she is!”

Although Monroe loved being photographed, Williams’s own real-world experience with photographers has been torturous. After the death of Heath Ledger, her onetime romantic partner and the father of her daughter, Williams was stalked by paparazzi. Asked whether her experience with media scrutiny prepared her at all for playing the world’s most-watched woman, she takes a long pause. She speaks about how, for a time, the relentless pursuit made her question whether acting was a career conducive to raising a child who was “a good person, a happy person.”

She says, surprisingly, that she didn’t really consider the correlation between her life’s experiences and Monroe’s. But she suggests that having survived that period without abandoning acting may have left her “willing to take on a big challenge.” As modest as she is, Williams seems satisfied with the result.

“If I had seen myself back in the early days when I was working on my Marilyn,” she says, “if I had seen how bad I probably was, I don’t know if I would have been able to go forward.”

But Williams compares her own learning process to her daughter’s lovely but awkward first stabs at making words of squiggles on paper. “It’s important to allow yourself to make mistakes, and to not get too hung up on them. It’s good to remember that the beginning of everything is ungainly and shouldn’t be seen.”

After all, sometimes those ungainly attempts at self-invention produce a Marilyn Monroe.

DeFore is a freelance writer.

“My Week With Marilyn” opened Wednesday at area theaters.