Playing dress up isn't all fun and games.

Just ask Barbie's costume designer Jacqueline Durran, who revealed that the most challenging part of her job was dressing Margot Robbie as the iconic doll in Greta Gerwig's upcoming movie.

"Margot is just about the most perfect human you can imagine, so she didn't make it hard," Jacqueline exclusively told E! News while promoting her partnership with ThredUp. "What was hard was narrowing down all the options and making sense of all the possible looks we could go for."

Considering Barbie is known for her keen sense of style, there was no shortage of glitzy and glamorous getups displayed on-screen. In the film, Margot wears everything from a sailor blue romper to a blush-colored gingham dress to a pink jumpsuit reminiscent of Rosie The Riveter's famous 1942 look. 

But the fact that Barbie has an outfit for every occasion proved to be difficult.

"The other big challenge was that Barbie changes her costume every time we see her," Jacqueline noted, "so there was a lot to do in very little time. We only had 11 weeks of prep time before the shoot started!"

But putting together the ensembles for the film is basically what childhood dreams are made of.

"My main inspiration was the Barbie catalog," Jacqueline revealed. "I tried to do a comprehensive sweep of different Barbie looks. One example was all the white and gold Barbie costumes from the '60s to the '90s for the block party. I chose the strongest looks with the best hair ornaments and lots of frills, particularly ones that were uniquely Barbie."

She continued, "Creating the fashion for this movie was so special, and I wanted to make sure I honored the rich history of the doll and its many style iterations over the years."

Jacqueline also drew inspiration from 1960s beach culture, replicating the whimsical color palettes and dress shapes of the era. "All the Barbies wear retro dresses of different designs," she said, noting, "The early '60s style pink gingham dress is one of my favorites."

And while it may have been challenging to figure out which pieces to narrow down, Jacqueline made sure that every item felt true to Barbie.

"Barbie's wardrobe comes from her love of clothes," she explained. "She wears outfits because she enjoys dressing up and always dresses perfectly for whatever she is doing. Barbie's style is playful and full of joy. It is over the top and, perhaps, looks superficial but it's actually a big part of her character."

It's clear Barbie's passion for fashion has translated off-screen, as the outfits have taken a life on their own with the Barbiecore trend. As Jacqueline put it, "It's been a joy to see how the costumes have inspired people to create their own looks and the movie isn't even out yet!"

Seeing people run wild with their imagination is part of the reason the costume designer teamed up with online consignment store ThredUp on a Barbiecore Dream Closet collaboration, which features more than 250 items.

"I hope that my partnership with thredUP and the costumes themselves remind people that fashion can be fun," she shared. "As a costume designer, I am a huge believer in the power of clothes and the stories they tell in film. With Barbie, there is no exception."