Discussion about shot timing, sequence and direction generates endless controversy because there is so much conflict between witness data and the photographic data that cannot be easily reconciled. This has resulted in two schools of thought: the Zapruder film is authentic, and the Zapruder film is a fake. Here is an article which frames the two points of view.
Here is an argument for fakery,
and here is an argument for authenticity.
Those who argue that the Zapruder film was faked must give some explanation as to how it was done. Since the Zapruder film was copied into several versions on the day of the assassination, the alteration had to have been done early before the copies spread too widely. Doug Horne gives an explanation for when the film might have been altered during the assassination weekend.
As to how it was altered, if it was, there are two methods: one is to excise frames from the film. For example, the Zapruder film starts with the lead motorcycles turning from Main to Elm and at Z frame 132, there’s a jump in the film to Z133 where JFK’s limo is already on Elm.
Zapruder never said that he turned his camera off and on. An edit could have been done to hide the limo turn from Houston onto Elm, which was by witness accounts executed poorly. Interestingly enough, Robert Harris notes that the Towner and Hughes films were damaged while in custody of the FBI so they don’t show the turn onto Elm either. Harris posits that this was to hide a first shot in the intersection. An alternative reason to hide the turn onto Elm is to hide an error by William Greer, the driver of the presidential limo. According to Roy Truly, "the driver of the Presidential car swung out too far to the right, and he came almost within an inch of running into this little abutment here, between Elm [the Elm Street ‘extension’ that runs in front of the Texas School Book Depository to the railroad parking lot] and the Parkway [Elm Street that goes under the triple underpass]. And he slowed down perceptibly and pulled back to the left to get over into the middle lane of the parkway. Not being familiar with the street, he came too far out this way when he made his turn.” Perhaps this was just an innocent mistake, but it is curious that just a few minutes before the motorcade arrived, an ambulance had been parked at that exact spot, picking up a person having an epileptic fit. The ambulance left just before the motorcade arrived. The ambulance driver, Aubrey Rike, later told people that FBI agents had called on the ambulance company following the assassination, apparently interested in records recording that the ambulance company had received at least half a dozen false calls requesting an ambulance to the corner of Elm and Houston in the week before the assassination. The calls were all made at mid-day and requested an ambulance for “right in front of the fountain” at the intersection. The call for the ambulance was made at 12:19pm. The motorcade was running about 10 minutes late; if it had arrived at 12:20pm, the driver of the limousine might have had to come to a complete stop, which would have made the shooting much easier.
In What Happened on Elm Street? page 79, over twenty witnesses reported that the presidential limousine stopped or almost stopped around the time of the head shot. See these reports from four policemen:
James Chaney (motorcyclist on motorcade): “From the time the first shot ran out, the car stopped completely, pulled to the left and stopped.”
Bobby Hargis (motorcyclist on motorcade): “The car stopped immediately after that and stayed stopped for about half a second, then took off.”
Earle Brown (police officer on overpass): “When the shots were fired, it (the car) stopped.”
J. W. Foster (police officer on overpass): “Immediately after Kennedy was struck… the car pulled to the curb.”
However we don't see that in the Zapruder, Nix, or Muchmore films. These films are synchronized and show the same thing: the limo slowing to about 8 mph prior to the head shot, then, after Clint Hill climbs aboard, the limo raced off. How to explain such discrepancy? If you cut out the frames which showed the car stopped, perhaps you could hide it. The problem with that is that if you cut out frames in the film, the resulting motion of the occupants in the limo or the background would be discontinuous, and if you look carefully at the film, it isn’t discontinuous, except perhaps for an overly quick turn of the head by the driver, William Greer between frames Z317-Z321 and a fast head snap by Mrs. Connally.
Some people think there was a big consipiracy to hide a shot from the front which blew out a big hole in the back of JFK's head. Such a head wound is not seen in the Zapruder film, but a large wound on the right side of his head, which looks like a blob is seen. Conspiracy researchers like David Lifton view think that the films were altered by painting on each frame, as in a cartoon animation. Others just think this is what it looked like from Zapruder's perspective. We'll look more closely at this in Chapter 3: Medical Evidence.