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 1964 Cadillac hearse that transported John F. Kennedy's body from the hospital to Love Field is almost exactly the same as the original one, down to the license plates (WC609). The original sold at the Barrett-Jackson auction for $160,000.
The movie was based on Vincent Bugliosi's book "Four Days in November: The Assassination of John F. Kennedy". That book, as noted in the credits, was an excerpt from his book "Reclaiming History", in which he takes a prosecutorial approach to prove that Lee Harvey Oswald carried out the assassination alone, and systematically debunk conspiracy theories. He began the book after serving as the "prosecutor" in the British documentary On Trial: Lee Harvey Oswald (1986), which conducted a jury "trial" of Oswald. Producers Tom Hanks, Gary Goetzman, and Bill Paxton originally bought the rights to the book intending to produce it as a big-budget HBO miniseries. This film is a compromise of the project they had in mind.
Abraham Zapruder films while standing on a column, with his secretary Marilyn Sitzman standing near him. In real life, Zapruder suffered from vertigo, and his secretary stood with him atop the column to help him with balance.