In an interview in TV Guide in the 1970's, Ken Howard related that he was rehearsing some of his lines from the play aloud while traveling on an airplane. The inflammatory revolutionary rhetoric caused the plane's crew to summon the police and he was questioned by the authorities when the plane landed.
All of the exchanges between John Adams and Abigail Adams are based on the real letters they wrote to each other while John was away. He called her his "dearest friend" and their letters ended with "Til then".
According to the writer/director's commentary, John Adams' actual quote following Benjamin Franklin's urging to remove the slavery clause from the declaration was, "If we give in on this issue, there WILL be trouble 100 years hence." The first battle of the Civil War occurred 85 years later, in 1861. The commentary stated that the quote was left out because it sounded too much like hindsight.
While it is a running gag of the film that John Adams is considered "obnoxious" and is "disliked" by the other members of the Continental Congress, in David McCullough's Pulitzer Prize winning biography "John Adams" McCullough said he had examined the written recollections of all the members of the Congress and none of them had anything but praise for Adams--except for John Adams himself.
Ron Holgate did all of his own riding (except for the trick mount at the end) in "The Lees of Old Virginia," even though he'd never been on a horse before.