In a new Entertainment Weekly profile to commemorate the film's approaching 15-year anniversary, director David Frankel revealed that McAdams, 42, had actually turned down the part three times before it was actually offered to Hathaway, 38.
According to Frankel, Fox 2000 - the studio behind the movie - wanted an established actress and McAdams was their first choice due to her success in The Notebook and Mean Girls at the time.
"We started negotiating with Annie to make a deal, and that didn't go well with the studio.… We offered it to Rachel McAdams three times," he recalled. "The studio was determined to have her, and she was determined not to do it."
"It spoke to me. It made me feel. It was about a subject that I take very seriously, but in such a wonderfully joyful and lighthearted way," Hathaway told EW.
20th Century Fox/Kobal/Shutterstock Anne Hathaway in The Devil Wears Prada
To convince the studio, Hathaway met with executives and relentlessly campaigned for the part, even writing the words "hire me" in the zen garden of then-Fox 2000 vice president Carla Hacken.
The studio started seriously considering Hathaway after Meryl Streep - whom herself was nearly not cast in the film as vicious fashion editor Miranda Priestly - put in a good word for the actress.
"Meryl was eager to make the movie, and she said 'let me meet with her.' Brokeback Mountain was about to come out. Annie had a wonderful, small role in that. And Meryl watched that scene from the movie, she met with her and called up Tom Rothman at Fox and said, 'Yeah, this girl's great, and I think we'll work well together,'" Frankel said.
When the studio finally offered her the part, Hathaway was beyond ecstatic.
"I patiently waited until it was my turn, and I got the call. It was the easiest yes in the world," she said. "I remember the moment I found out I got the part, I just ran screaming through my apartment. I had a bunch of friends over at the time, I just jumped up in the living room and screamed, 'I'm going to be in The Devil Wears Prada!'"
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Reflecting on her own iconic role, Streep, 71, told EW that she attempted method acting to play the icy Priestly - something that she did not enjoy in real life.
"It was horrible! I was [miserable] in my trailer. I could hear them all rocking and laughing. I was so depressed! I said, 'Well, it's the price you pay for being boss!'" the three-time Oscar-winner remembered. "That's the last time I ever attempted a Method thing!"
Everett (from left to right) Anne Hathaway, Meryl Steep and Emily Blunt in The Devil Wears Prada
Hathaway admittedly "did feel intimidated" by Streep on the set, "but I always felt cared for," she said.
"I knew that whatever she was doing to create that fear, I appreciated [because] I also knew she was watching out for me. There's this scene where [she says], 'You're just as disappointing as the rest of those silly girls,' " Hathaway said. "I remember when the camera turned on me, the pressure really got to me, and I'd had such emotional fluidity in the day up to that point but it just wasn't there anymore."
"I remember having the experience of watching [her] watch me, and [she] altered [her] performance ever so slightly, and just made it a little bit different, and brought more out of me and got me to break through whatever barrier I had," she added.