The picture from 22 November 1963 with Clint Eastwood's head circled in red ink, is actually an edited, blown-up version of an original photo taken by James 'Ike' Altgens. In the original photo, only agent Clint Hill is looking forward. This photo has been a controversial point for researchers, as many people say you can see Lee Harvey Oswald in the doorframe of the Book Depository. But, in actuality, the person mistaken for Oswald is fellow Texas Schoolbook Depository worker, Billy Lovelady.
Two of the actors in the film held public office - Clint Eastwood served as the Mayor of Carmel, California (1986-88), and Fred Dalton Thompson (who in real life is an attorney and lobbyist) - he ran for the U.S. Senate to fill the unexpired term of Al Gore (at the time of filming was the Vice President) a year later and re-elected to a full term in 1996. He later declared his candidacy for President of the United States in 2007, but withdrew his candidacy January 22, 2008.
When the agent is checking the pack of photos against guests arriving for the dinner, Mitch's photo is a single frame. He then looks at Mitch, doesn't recognize him, and shuffles the now two-frame photo to the bottom of the pack.
When we see Leary's view through his binoculars, his view forms the figure eight, but if binoculars are correctly adjusted, they form a circular view. This is a common error in movies, and is often made deliberately in order to show the viewers that now we see the scene through binoculars.
What are the differences between the British BBFC 15 Version and the Uncut Version?
The old British BBFC 15 versions are all cut by approx. 8 seconds. Several death scenes were affected, e.g. the killing with a plastic bag, the breaking of necks and a shootout. All cuts were waived for the Blu-ray release. A detailed comparison between both versions with pictures can be found here.
How did Horrigan know the gun was unloaded in the scene where the gangster try to make him shoot his partner?
An empty pistol is considerably lighter than one with a clip loaded, an untrained civilian might not be able to tell the difference, but someone with proficient firearms training can. This is not an exact science, however, as it does not account for chambered rounds. Horrigan could tell by the weight that there was not a clip in the pistol. There could have still been one in the chamber, but his instincts suggested there was not, and in this instance, his instincts proved right. He also thumbs back the hammer allowing him to see there is no round in the chamber.