Carey Mulligan revealed that she had studied the love letters from Ginevra King, F. Scott Fitzgerald's first love, and Zelda Fitzgerald, the writer's infamous wife, in order to create Daisy's character. She said, "In the novel, there are lots of holes in Daisy's story. If you read the novel, there are lots of moments where she isn't saying anything." So, Carey and Baz Luhrmann created new lines for her using the letters. Baz said that Carey would come up to him in a scene and mention that it would be a perfect moment to slip in one of the lines from Ginevra's or Zelda's letters, which he admitted "just worked" with the film.
Catherine Martin, Baz's wife and costume director for the film, also offered up her interpretation of Daisy and why she found her difficult to relate to as a modern woman. She said, "Daisy's a product of her era and her sociological milieu. When she talks about her daughter and says that she hopes that she's grows up to be a beautiful little fool, because that's the best thing she can be, a beautiful little fool, that's the moment you realise Daisy isn't a fool, but she's trapped within her own circumstances. You understand her; you feel pity for her. You know, not everyone is [Daisy's friend in the film] Jordan Baker, not everyone is built for the modern world, and Daisy was basically born and bred to be a trophy wife, but she's smarter than that."