After Natalie Portman was cast, to Pablo Larraín's wishes, he asked screenwriter Noah Oppenheim to tear out any pages of the script that didn't contain scenes with Jackie Kennedy, as he wanted this movie to be entirely about her and her experiences. The 120-page script was trimmed to one hundred pages, all containing Jackie.
One challenge that loomed for Natalie Portman as she prepared was Jacqueline Kennedy's highly distinctive dialect, impeccable diction, and whispery voice. "She had such an amazing voice", Portman mused. "It was truly from another era. She had a finishing school sort of way of presenting yourself, very demure, where you bat your eyelashes and speak in a breathy voice. Her accent was posh, but also mixed with a real New York accent, and also a little British. Her dialect is an unusual combination of sounds that were completely unique to her. The first time I did it on set, I think Pablo Larraín was terrified", Portman recalled.
The real pink suit worn by Jackie Kennedy when John F. Kennedy was assassinated is currently held in the archives in Maryland. According to family wishes, it will not be put on display for the public until 2103.
Billy Crudup's character, billed as "The Journalist" in the credits, was meant to be Theodore H. White of LIFE Magazine, who conducted a real-life interview with Jacqueline Kennedy.