After discovering that the adaptation was in the works, Saoirse Ronan reached out to Greta Gerwig and told her she decided she would play Jo March. Gerwig was initially hesitant to cast Ronan after having just worked with her on Lady Bird (2017), but after realizing that casting herself was, more or less, a very Jo thing to do, Gerwig sent Ronan an e-mail that said, "Yes, you're Jo."
Amy's significant speech about marriage was not part of the original script. It was suggested by Meryl Streep. After collaborating with the writer and director, Greta Gerwig, Meryl Streep emphasized the need for a moment in the film that resonated with modern audiences, showcasing the true powerlessness of women during that period. Women not only lacked the right to vote and work, but through marriage, they would also lose ownership of their money, property, and children. According to Florence Pugh, Streep received her lines just a few minutes before they were set to film the scene, and they were handwritten on a scrap piece of paper.
Florence Pugh had just finished filming Midsommar - Il villaggio dei dannati (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.
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.