Ultimately, she did take home the Academy Award -- and praise from fashion critics for her blue Gucci gown.
Her stylist, Cristina Ehrlich, told ABC News that the look was two months in the making.
"The initial part of the inspiration definitely started with picking the right designer, and picking the right designer for her didn't necessarily mean that we had a color set or anything set. It was kind of about figuring out the vibe of what wanted Oscars to be for her," Ehrlich said.
She continued: "[Gucci creative director Alessandro Michele] is an artist and a visual man, and Brie is absolutely too an artist and incredibly visual, so it was a dream collaboration. And I think for her, not only was it the end to an outrageously successful season, but I feel like it also really opened her up to this journey of understanding and learning how she can play these different characters on the red carpet."
Ehrlich first met Larson, 26, last May, when they worked together on her Met Gala look. However, when it came to awards season, the stylist told ABC News that she and Larson knew they wanted to stay away from black and white, and focused on blue and pink. Ehrlich did have a few reservations about the latter, though.
"Pink can either feel very sophisticated or it can really suggest like a Cinderella princess," she explained. "Brie is so sweet and has such a big smile and is such a happy girl -- I didn't want it to get too young and saccharine."
Ultimately, three designers submitted ideas months ahead of time. The day before Christmas, they committed to Gucci.
"I had individual conversations with the brands about what we wanted the overall feel to be and what we wanted the sensibility to be, and the sketches that Gucci provided for us really, really hit home for Brie and I across the board," she explained. "They tapped into some of the stronger elements of her personality and what she had fantasized the Oscars to be like."
However, several elements changed as time wore on. For starters, the dress was not originally meant to be blue (Ehrlich demurred when asked what color the designer first envisioned) and noted that the skirt in the sketch was much more ornate than the one on Larson's final dress. The belt, a focal point of the dress, changed a lot too.
"Originally there was a strong element [that] suggested of some type of a flower being incorporated," Ehrlich said. "It was a big, beautiful flower that would go around the waist, but we decided if you looked in profile, it was too distracting."
As a backup to the dress she wore, Gucci also created a second, more fluorescent blue gown for Larson, though it wasn't needed. And, for the Vanity Fair after-party, Larson opted to wear a crushed velvet MONSE gown, which she paired, Ehrlich noted, with her own Converse sneakers.
"That's Brie!" she said with a laugh.
"I almost thought, 'Oh, maybe the luncheon with her pink blouse will be our pink moment,' but for Vanity Fair, [after I saw this dress] I told the designer, 'Mark my word. She will wear this.' I could just feel it," Ehrlich continued. "It was the first and only one she tried on. She put it on and said, 'This is the dress.'"
Everything was kept in great shape, and yesterday, Ehrlich noted, Larson sent back her Oscars gown to the design house in Italy.
"For this particular dress, and be it that Alessandro Michele is at the start of his helm there, it should be in their archives," Ehrlich said. "I wouldn't be surprised if she'd asked if they would have let her keep it -- but that's a testament to who Brie is -- and having her Oscar on the shelf is enough proof of the evening!"