*In a recurring series,*Vanity Fair pulls back the curtain on awards season’s most visually enticing films, revealing exclusive details of the creative process of art directors, costume designers, makeup artists, cinematographers, and more. This week, top prosthetics designer Mark Coulier explains how he turned Meryl Streep’s chiseled cheeks and slim nose into the rounder-faced resolve of Margaret Thatcher, garnering an Oscar nomination for best makeup. (Streep is also nominated for her performance as Thatcher.)

In The Iron Lady, Oscar-nominated Meryl Streep embodies Margaret Thatcher so wholly that their physical resemblance, even in the prime minister’s younger years, appears to be achieved by spitfire alone. But if the actress takes home the Oscar for her 17th nomination, she—like Nicole Kidman before her—will have won by a nose. Streep wore a prosthetic piece in every scene of the film. Surprised? The Oscar-nominated makeup team behind the look couldn’t be more pleased. J. Roy Helland (Streep’s makeup artist for the past 30 years) and Mark Coulier (best known as Voldemort’s personal prosthetics designer) set out to channel Margaret through Meryl in the most natural way.

“We looked at hundreds and hundreds of photographs during our research for the project, trying to work out what were the best features to pick to fit onto Meryl Streep,” says Coulier. “There are several things that struck us initially. At the top of Meryl’s nose we created an arced piece, on the inner brow of her nose. Meryl’s got quite a thin [nasal bridge] really, so she looked completely different. We blocked it out on the cast and we said, ‘That really does make a difference.’ We didn’t do the whole nose, purely to avoid commentary. Meryl was quite delighted that no one really noticed it.”

Next, Coulier worked to soften Streep’s signature striking bone structure. “Meryl has great bone structure. Really good cheekbones.Thatcher’s got a rounder face,” he says. He also introduced dentures to push out Streep’s top lip and “create that slight overbite that Margaret Thatcher appears to have.” Since the film spans decades, during Thatcher’s school years a younger actress (Alexandra Roach) plays her, and so Roach got the nose and dentures, too.

Given Coulier’s deep bag of tricks, his restrained decision to mimic just a few of Thatcher's features is in itself a notable choice. But the designer says it’s a fine line between illusion and distraction, particularly with a famous face. (Look, for example, at what critics said about J.Edgar, with the heavier-handed makeup its artists applied to Leonardo DiCaprio.) One place where Coulier decided less was more was Streep’s eyes: “Margaret Thatcher has these distinct hooded eyelids,” he says. “We played around with the idea and it worked, but we just decided that we didn’t need it…. I thought back to other films. Will Smith played Muhammad Ali really effectively and he looked nothing like Muhammad Ali. Anthony Hopkins played Nixon. If you can rely on the performance, you don’t have to go down that route.”

Impressively, in a biopic that spans some 60 years, there’s never a moment when Streep isn’t convincing at Thatcher’s age. As her political career takes off at age 34, the 60-something actress is luminous. “That’s what Roy does so well. It’s his job to make Meryl look beautiful,” says Coulier. “Makeup, powder, lighting, all combine for the younger Thatcher. When we did the older Thatcher, we left all that stuff out.” In middle age, the designer says, they enhanced Streep’s fine lines and painted in wrinkles with makeup.

But it’s in the prime minister’s elder years that the team really had fun. Given a limited time frame to age Streep—“We did it two hours and 20 minutes, pretty much bang-on every time”—Coulier says they had to prioritize.

“We did her hands in a much more traditional way, because it would have added another 45 minutes,” Coulier says. “We used makeup, and painted liver spots all over her hands. Meryl did a very clever little thing, as well—she put elastic bands on her wrists to make her veins swell. We managed to make her hands look pretty old. Then for certain close-ups they used a hand double.”

Coulier and Helland considered employing a bald cap to mimic an older woman’s thinning hair, but ultimately they decided to simply pull back her hairline and concentrate on capturing the politician’s face and neck. “Looking at photographs, we noticed Margaret Thatcher in old age had quite an asymmetry in her face. You really noticed it on her nasolabial folds on the sides of her mouth—they’re different, one side to the other—so we put that into the film,” Coulier says. “To age her neck, we created these two wattle lines.”

Coulier molded the prosthetics out of silicone, as the material is soft and responsive, and worked closely with Streep to achieve a look that would suggest Thatcher without impeding the actress’s ability to become her. “Some actors really don’t like wearing prosthetics, but I think Meryl—well, likeis probably not the right word—but didn’t hate it,” he says. Even still, he adds, “The first tech [rehearsal was] pretty nerve-racking.” Fortunately, it all came together in the end: “We were just standing outside the makeup room, when this old lady wandered out of the costume department,” he says, of seeing her for the first time in costumes and makeup. “Even I took a little double-take and said, ‘Wow. That’s Meryl.’ It was pretty exciting—at that point, we were quietly confident that we would be able to pull it off.”