"I'm a notorious pain in the butt for any costume designer because I have so many opinions about how my people should present," she admits. "I feel very strongly that we make decisions about what we're giving to the world, what we're withholding from the world by virtue of what we put on our bodies, and what we choose to say and not say. So, for me, clothes are kind of character; they're more interesting in those terms."
The Oscar winner worked closely with Emmy-winning "Sex & the City" costume designer Patricia Field to create Miranda's vast, expensive and flashy ensembles.
As the editor-in-chief of Runway magazine, Miranda must look impeccable and put together at all times. A montage in which she tosses her purse and coat onto the desk of her newest assistant, college grad Andy Sachs (Anne Hathaway), over the course of several days highlights the need for a large volume of quality clothes and accessories.
"Pat had to make it by relying on her many good relationships with designers and talk them into loaning us stuff," says Streep. "People were very, very generous in the fashion business. I had 60-some costumes, and each one of them had to be coordinated with the shoes, the belt, the earrings, the jacket."
Looking perfect from head to toe also requires a head-turning coiffure. Miranda's wavy white hair conveys power, decisiveness and style.
"That was a decision of Roy Helland, my hairdresser and collaborator of many years," she explains. "We knew we wanted to make a definite kind of look, a woman that doesn't look like anybody else in New York. At the fashion shows, it'd be easy to spot her and look at her. I've always admired this model named Carmen. She's got a big swoop white hair and it appears to be real, natural hair -- that kind of defiance in the face, when you're just so over the top of everything, that you can just not dye your hair."
In contrast to Miranda's statement-making hair and clothes, Streep's performance as the demanding editor is carefully understated and deceptively mellow.
"I was just interested in making a human being as contradictory and messy as we all are," she says. "I think she is an exacting, highly disciplined demanding, ambitious person who doesn't necessarily take the time for all the nice social lubricants that help make the work place grateful and fun."
It's widely accepted that "Prada" author Lauren Weisberger based Miranda on real life Vogue editor Anna Wintour, but Streep looked elsewhere to get inspiration for her character.
"I know the book was based on an assistant's view of Anna Wintour, but it didn't interest me to do a documentary on Anna Wintour and I don't know anything about her," she says. "It's much more fun to make the uber boss out of my own pastiche of experience. Most of my models for this character were of the male end of the species. Miranda is so well behaved. She's almost like a diplomat compared to some people who are very, very powerful in our business. But I really understand her, admire some things about her, and see the bind that she is as a woman."
Miranda's ruthless business choices make her a successful, but ultimately lonely person, something Streep unfortunately experienced firsthand on the project.
"I didn't stay in character when they yelled 'Cut,'" she says, "but I also found I couldn't enter into this fun ... because I felt it wouldn't help the dynamic of the set if I immediately went over and was joking with [the cast]. They were always having a party in the corner. I just couldn't join in and it was sort of a lonely position. I staked out for myself. I suppose it paid off ultimately but it just wasn't that much fun."
"The Devil Wears Prada" opens nationwide on Friday, June 30.