‘The Devil Wears Prada’ holds up 15 years later because Meryl Streep was the mastermind behind Miranda Priestly

Fifteen years ago, Meryl Streep's award-winning role as Miranda Priestly in "The Devil Wears Prada" redefined what it meant to be a woman in power.

"It was the middle and the end of an era simultaneously. It was the height of that world in New York. And it was also the last gasp of that world," director David Frankel told Insider of the 2006 film, which follows naive journalism-school graduate Andy Sachs (Anne Hathaway) as she navigates a demanding assistant job under Miranda, the editor-in-chief at a fictitious-yet-heavily-Vogue-inspired magazine.

"Condé Nast and 400-page magazines and town cars for the assistants, all that stuff is a relic of history," he said. "It's a long time later [but] people are still watching it. And it's not anything that any of us anticipated ever."

In honor of the movie's anniversary, Insider spoke to Frankel about the unexpected challenges of making "The Devil Wears Prada" and why it wouldn't have worked without Streep.

The scrambling that occurred every morning when Miranda arrived at the Runway office wasn't dissimilar to the frantic activity behind the scenes of "The Devil Wears Prada."

Much of the film was the result of happenstance, beginning with casting. Hathaway previously said that she was the ninth choice for Andy, but she was actually "the first and only actress that I met with for the role," according to the director.

Frankel said that "negotiating stuff got in the way" with Hathaway early on. By the time they cast Streep, who was first to sign on, other actors like Scarlett Johansson and Kirsten Dunst were being considered for the inexperienced assistant.

"The studio was very excited about Rachel McAdams, for sure," Frankel admitted. "And she just did not want to do it. I think she got offered it three times over the course and she just kept saying no.

"At that time, it would be easy to perceive the movie as a kind of simplistic 'Cinderella' tale and she wanted to do more sophisticated work than that," Frankel said of McAdams, who was then known for roles in "Mean Girls" and "The Notebook." "I've talked to her well after the fact and she kind of shakes her head."

Ultimately, Streep put Hathaway back into the conversation with the studio, 20th Century Fox.

"She saw Annie's performance in 'Brokeback Mountain' in a supporting role and said, 'Hey, this girl's fantastic. What's the problem here?'" Frankel recalled of his conversation with Streep.

Miranda's other assistant Emily was also challenging to cast since the character was initially intended to be American.

Once Frankel realized Emily "somehow sounds more polished and ingratiating if they're British than if they're just sort of snarky American," he said he got ahold of Emily Blunt's audition tape and knew the Brit was perfect for the role. The problem was that she was screen-testing for the 2006 film adaptation of the fantasy novel "Eragon."

"They had her in first position, so we had to wait to see if she got that role," Frankel said, explaining she couldn't possibly nab both jobs because the movies were set to film simultaneously.

"At the very end of that process, she did not get the part in 'Eragon' and I was celebrating," the director recalled. "I called her that night and she was already drinking, drowning her sorrows out on the town. And, I said, 'Don't worry. The worst day of your life [is the] best day of your life.'"

Studio executives thought Blunt's performance was "so terrific" that they asked for an additional scene to be written for her character.

Originally, the last time Emily was seen was when she was bruised and lying in a hospital bed, yelling at Andy for accepting the coveted trip to Paris Fashion Week.

"The studio said, 'We can't leave Emily like that. We have to see her one more time later in the movie,'" Frankel said.

"For various reasons we kind of resisted," he added, explaining that it "seemed unauthentic" to add a scene near the end of the film. Eventually, they conceded and the additional scene neatly tied up loose ends as Andy calls Emily, offering clothes from Paris. Emily then goes off to tell the new assistant she has "very large shoes to fill."

Finally, rounding out the all-star cast was Stanley Tucci as Nigel, one of Miranda's closest colleagues at Runway with the film's best one-liners. He, too, was a tough one for Frankel to lock down, and ended up being cast "five or six weeks" after the movie began filming.

"I probably met 150 guys to play Nigel and had sort of very specific criteria that we were aiming for," Frankel said about the character, who needed to be smart and sophisticated. "For lots of reasons, Stanley just wasn't a part of that conversation for a long, long, long time."

"The studio was freaking out because the part hadn't been cast," the director recalled. "It was like on a Friday and the part worked on a Monday. I went to lunch with Stanley and I loved him."

Frankel said that initially "it wasn't clear" the film was working.

Since the movie was adapted from Lauren Weisberger's novel of the same name (based on her experience as a personal assistant for Vogue editor Anna Wintour), it was difficult to produce - from getting the fashion industry to support the film to finding glamorous locations for the set.

"The book was obviously not welcome in Condé Nast circles and anybody who wants to stay on the right side of Anna Wintour just did not want to participate," Frankel said. "There were a lot of people we asked who turned us down or just said, 'I'll give you my two cents. I'll read your script. I'll be an anonymous source.'"

Frankel did manage to land cameos from designer Valentino Garavani (who agreed 24 hours before showing up on the set) and models Heidi Klum and Bridget Hall. Gisele Bündchen even played a minor role as an employee at Runway.

They also had a tough time securing designer looks for the film's cast, until Prada, whose name is in the movie's title, gave in and let costume designer Patricia Field borrow whatever she wanted.

Filming on location in New York City came with its own logistical challenges, too. Frankel said that, even 15 years ago, it was difficult to secure an apartment that fit their vision of how Miranda lived.

"And in fact, we never did find it," Frankel admitted.

A friend of a producer let them use a then-newly renovated townhouse on the Upper East Side. But as a result of the home's layout, they had to rewrite the scene in which Andy botched her delivery of the Runway mockup book that Miranda reviews every night. Rather than walk into the wrong hallway, Andy went up the stairs, which turned out to be an even bigger transgression.

Andy's makeover from baggy cerulean blue sweaters to Chanel boots also posed a scheduling issue that the crew didn't anticipate.

"We would shoot a scene where she's pre-makeover and then we just sat there for three hours while she went to get her hair done and then she would come back," Frankel said.

Eventually, they realized that it was easier to move the whole crew to the next setting than to wait hours for Hathaway.

Despite the hardships, Frankel put together a promising sizzle reel for Fox 2000, based on the film's early footage.

"It was obvious that whatever we were doing, even as painful as it was to make it, that it was working," Frankel said. "And the studio then gave us a real confidence boost."

The studio was so trusting in the film that they scheduled it to open during the same weekend as "Superman Returns," a movie expected to dominate the summer box office.

"We just sailed through the second half," Frankel said. "You just kept waiting for the other shoe to drop and it just never did. It was one of those lucky, blessed movies."

Frankel always viewed Streep's character, with her pursed lips and bone-chilling "that's all," as the heroine of the story, not the villain.

But Weisberger's novel was more about the personal assistant getting even with their boss.

"I just felt like that was unsatisfying," Frankel said. "It was unemotional for me. I feel like that leaves you kind of cold, someone getting their revenge."

"She held people to a higher standard and she sacrificed her personal life. She dedicated herself to doing something that was really great and pushed people beyond what they felt they were capable of," he said of Miranda.

"I admire that in people and I wanted to make her the hero of the movie," he added.

Frankel even admitted that part of the film's enduring allure is that it centers on a tough boss, but one that's not necessarily toxic.

"For sure she's tough and demanding and mercurial and seems mean at times. But at the end of the day, it's not cruel and it's in the dedication toward excellence," he added. "It isn't a power play. It isn't machismo. It isn't uncontrolled anger."

Streep helped craft her now-iconic role, often giving notes throughout filming to revise speeches or tweak her character's appearance.

Even Miranda's quiet but deadly voice was intentional. It was inspired by Mike Nichols and Clint Eastwood, who Frankel said were her "favorite directors."

The version of Miranda seen in the film aligned more with Frankel's perception of the character, and was largely crafted by Streep and screenwriter Aline Brosh McKenna. Frankel said that Streep ultimately "deserves all the credit for every ounce of Miranda Priestly."

One scene, in particular, was "100% Meryl," according to Frankel. The moment focused on a makeup-free Miranda struggling to keep her personal life together while still remaining at the top of her game career-wise, and being very aware that her latest divorce would be tabloid fodder.

"We were shooting at the St. Regis Hotel in New York and they said, 'Meryl's ready,'" Frankel recalled. "And I got in one elevator and we got off the elevator about the same time in the hall and there she was. And I saw her and I was shocked. I was in shock as the audience is when they first see her like that. "

"In a moment, you get it, what she was going for and she did it beautifully," Frankel said, adding that they could only shoot the scene in a limited number of takes "because she was working so hard. It was such a dark place for her as an actress."

To reach those depths of authenticity, Streep applied a method-acting approach, keeping off-camera chatter with the cast to a minimum and never hanging out with Hathaway or Blunt.

"She definitely kept her distance," Frankel said. "She was very aloof and they were properly intimidated by that. It meant that they were always super prepared and always apologetic just to her as an actress, and that definitely fed all of their performances. She was able to properly condescend to them."

While "The Devil Wears Prada" never topped the box office upon its release, it did exceed Frankel's expectations.

The movie stayed in theaters longer than he anticipated and performed well overseas. On a budget of $35 million, it earned $124 million domestically and $201 million internationally.

"The fact that it crossed over, that it wasn't just for women, that was what made it such a success in the US," Frankel said. "And then the international success of it was a huge surprise to me. Maybe the studio anticipated that, but not on that scale."

Like any successful movie, there were discussions about a potential sequel, both immediately following release and in 2013 when Weisberger put out another book set years after the first novel.

Frankel said the team considered doing another movie for "like five minutes," but decided against it because they told the story that they originally set out to.

"And I think that was the right decision," he added.

"Aline always considered that this was the story of somebody climbing Mount Everest," Frankel continued, referring to the screenwriter. "Miranda is Mount Everest and Andy is making her way to the peak. And it's like, 'OK, we got there.'"