The Devil Wears Prada costume designer Patricia Field on Andy's style evolution

The Devil Wears Prada may have come out in 2006, but many of the looks featured in director David Frankel's film about a newly hired assistant, Andy Sachs (Anne Hathaway), to a fiercely intimidating editor, Miranda Priestly (Meryl Streep), feel as fresh and fashionable as ever. Here, costume designer Patricia Field (Sex and the City, Ugly Betty) tells EW about Andy's style, going through her evolution from fish out of water to extraordinarily à la mode to somewhere in between.

Andy could not be more out of place when she first enters the offices of Runway, a fashion magazine with employees who look like they walked off its chic pages. "At the beginning, she's not at all clued into fashion," Field says of Andy. "She wasn't so conscious about how she was dressing, which made it fun because we started out with this, let's say, basic, typical way of dressing, like a Gap or J. Crew kind of look. As she [lives] in this fashion world, she's under pressure to adapt, and I think she wanted to adapt because the world around her was dressed up and dressed differently."

One of Andy's most memorable looks (for all the wrong reasons) is her cerulean sweater that becomes the target of a verbal attack by Miranda. "It's sort of comical how you think that you've made a choice that exempts you from the fashion industry," Miranda says of Andy's choice to wear said sweater, seemingly because she takes herself too seriously to care about her clothes, "when, in fact, you're wearing a sweater that was selected for you by the people in this room from a pile of stuff." The scene is still famous all these years later, as evidenced in part by the fact that people frequently mention it to Field. "That's the kickoff of [Andy] realizing she had to get with it and not be so nonchalant about her work," she says. "It was a very important scene, and Meryl, she delivered it like the pro that she is."

Soon enough, Andy does get with it, and she does so in a big way. Her first outfit post-makeover (thank you, Runway closet!) is entirely Chanel, including those standout boots. "The reason I chose Chanel is because, number one, I felt Annie Hathaway was a Chanel girl organically, as opposed to, let's say, a Versace [or Roberto Cavalli] girl," Field recalls. "The other reason was I have a relationship with Chanel, and when I called them they were very happy to work with me on this movie because they wanted to see Chanel on a young girl to give it another point of view — [making] Chanel not just middle-aged women in suits, but youthful and funky." The help from the brand was huge for "a movie with nothing but wardrobe," but, most importantly, Field continues, "the clothing suited the girl."

One of Field's personal favorite looks is the green coat Andy wears while walking in the streets of New York. "I loved it because of the color: It was the first thing that you saw, it was striking, and I think it hit the mark," she details. "For me, the success of the clothing is not only that it looks good, but it has to tell the story. So when she's walking in these streets that are gray and gritty, she popped." That coat is part of a montage showcasing Andy's new wardrobe. To make the many looks featured in said montage stand apart, Field played further with color and avoided trends of the time. "I wanted it to remain in time and look classic forever," she says, "and that's my favorite montage for that reason."

For comparison, Miranda's style is very different from Andy's, and is very personalized to her. "I did not want to copy Anna Wintour or anybody," Field says. "I wanted to create Miranda Priestly, a unique style for her." To do so, Field turned to the Donna Karan archives, describing the designer as one who made clothes that were flattering, easy to wear, and sophisticated. "I didn't want to use anything recognizable. I thought that time had maybe erased the recognition of the new generation. Her clothes were so classic, [but] they didn't have a vintage look about them. It helped me to create her own style."

As for the style of the other assistant, Emily, the actress who played her was key. "I was very inspired by Emily Blunt herself because she's very strong and she carries herself very well," explains Field, who received an Oscar nomination for her work on the film. "She's got a great body, so I made her a little more edgy because she could handle it." Handle it, she did. "The person has to wear the clothes. The clothes cannot wear the character. Otherwise, it doesn't work. It has to be believable."

By the time Andy attends a gala with Miranda and Emily in a sleek floor-length gown, she's well adjusted to her new world. "She started to notice fashion, then she started to be more conscious of the way she dresses," Field says, adding that the montages, in particular, captured her new aesthetic and that the aforementioned Chanel miniskirt, thigh-high boot outfit marked a culmination of her old world and new one, the push and pull between her boyfriend Nate (Adrian Grenier) and very consuming new job. Case in point, "[She] went from a regular girl to a fashion girl."

Field was asked to — and ultimately did — design a purse, which Andy carries throughout much of the movie. The purse, Field explains, was a sort of vertical rectangle with a hole cutout up top as a handle. The brown, fringed accessory was somewhere between a bag and a tote. As for what Field wanted to convey through the purse? "Again, I wanted to get something original that we hadn't seen, something that had a little quirkiness."

Toward the end of the film, Andy wears an elegant green dress while in Paris on business. It's one of Andy's many outfits that still feels like it could be worn years after its June 30, 2006 debut. "I'm very happy to say that I think her style holds up," Field says. "I think that the style of the whole movie holds up." She continues, "If your originality is based in a classic, then you trim it with other layers that give it uniqueness. It's not a copy from a magazine or trends. Time is going to kill it very quickly if you go that route."

The last look we see Andy in is a brown leather jacket, a black turtleneck, dark jeans, and brown boots that fall just below the knee, altogether bridging the gap between her indifferent looks at the beginning of the film and her fashion-conscious ensembles from her time at the magazine. "She went in one girl, she came out another girl," Field notes of Andy, who quits working for Miranda while in Paris. "Rather than a compromise, I would call it a combination, because she knew that she couldn't go for another job that has nothing to do with fashion looking like she just stepped off the pages of the fashion magazine. I don't think that the people [interviewing her for the job] would necessarily get it, so I believe she thought about it and said, 'Okay, I'm not going in my cerulean blue sweater, but I want to get myself together a little bit here.'"