After finding out that the adaptation was in the works, Saoirse Ronan reached out to Greta Gerwig and told her she decided she was going to play Jo March. Gerwig was initially hesitant to cast Ronan after having just worked with her on Lady Bird (2017), but after realizing that more or less casting herself was a very Jo thing to do, Gerwig sent Ronan an e-mail that said, "Yes, you're Jo."
Florence Pugh had just finished filming Midsommar (2019), a few days prior to when she started shooting this movie. She said that getting to play Amy after making such a stressful and anxiety-inducing movie was her version of therapy.
Amy's big speech about marriage was not in the initial script but was suggested by Meryl Streep. After working and talking with the writer and director Greta Gerwig, Streep asserted that there needed to be a moment in this movie that allowed modern audiences to understand the true powerlessness of women in that period. Not only could they not vote or work, but through marriage, they would lose ownership of their money, property, and children. According to Florence Pugh, the lines were given to her a few minutes before they were due to shoot the scene. It was handwritten on a piece of scrap paper.
Each sister was given a distinctive color palette for their wardrobe: Meg's is lavender and green, Jo's is red and indigo, Beth's is pink and brown, and Amy's is light blue. Their mother, Marmee, often wears a combination of their colors.
Costume designer Jacqueline Durran made several duplicate costume pieces for Jo March and Theodore "Laurie" Laurence to make it seem like the characters share clothes.