The family of Abraham Zapruder, the garment-industry executive who shot the 26 seconds of 8mm film that recorded the instant of the shooting, had never publicly shared their story before. "This is the first time the family has cooperated with anyone," says Landesman. "I think they agreed to help because this script treats him with objectivity, clarity and fairness, perhaps for the first time. They realize this might be their last chance to tell their story themselves."
Dr. Earl Rose was actually on solid legal ground when he attempted to have Kennedy's body autopsied in Dallas, instead of the Secret Service removing the body. At the time of Kennedy's death, there was no federal jurisdiction over the assassination of a president. If Rose had prevailed, the chain of evidence would have been preserved. Since the Secret Service forcibly removed the casket, what happened to Kennedy's body between the time it left Parkland and arrived at Bethesda Naval Hospital has been the source of great speculation, and led to numerous conspiracy theories.
The idea for the film began in a conversation between actor, director and producer Tom Hanks, producer Gary Goetzman, and actor Bill Paxton. Goetzman said: "We were at a baseball game and the Kennedy [John F. Kennedy] assassination came up. Bill is fascinated by all of the theories about it. He is from Fort Worth and remembers seeing Kennedy speak there on the day of the assassination." Sometime later, Paxton visited the Sixth Floor Museum at Dealey Plaza in Dallas, Texas, USA which is dedicated to the assassination and legacy of President John F. Kennedy. Watching a film loop of the president's visit to Texas, he was shocked to recognize himself in the background, perched on his father's shoulders. When he stopped in an airport bookstore later the same day and found a copy of Vincent Bugliosi's book, "Four Days in November" (2008), he became determined to find a way to tell the familiar story from a particularly personal point of view. He shared the book with Hanks and Goetzman, who began to see it as a perfect project for Playtone, which had produced a number of award-winning mini-series for HBO based on pivotal moments in American history, including John Adams (2008), Band of Brothers: L'enfer du Pacifique (2010), and Frères d'armes (2001).
Dr. Robert McClelland, who operated on President Kennedy and Lee Harvey Oswald at Parkland Hospital on the respective days of their deaths, was not portrayed in the movie.
Director and screenwriter Peter Landesman spent three days with FBI Special Agent Jim Hosty before he died in June 2011. "To me, he is the most tragic figure, because he was doing his job, the job he loved, but he always felt that the FBI and others blamed him for the assassination."