The murder of Oswald by Jack Ruby was filmed on location in the actual basement garage of Dallas City Hall, where the real-life shooting took place.
Oliver Stone showed this film in December of 1991 to all of Congress on Capitol Hill. It led to the 1992 Assassinations Disclosure Act. This Act allowed the American public to see important documents regarding JFK's assassination in the period of 25 years (the original year for such disclosure was 2029). On 26 October 2017, the Trump Administration released a great part of the documents known as the "JFK Files".
Oswald's arrest was filmed in the real Texas Theatre where it happened. Money from the producers helped to restore the theater and keep it in business.
John Candy's (Dean Andrews') sweaty face during his talk with Garrison is real. Candy was petrified at the idea of appearing in a dramatic film with actors like Gary Oldman and Donald Sutherland. He sweated profusely throughout all his scenes.
Oliver Stone: Seen very briefly in the assassination re-enactment. Look very close for him as the Secret Service agent who runs towards the back of the limo after the fatal headshot.