Do you believe any frames are missing in this sequence? I set the animated gif to real time, which has Greer facing forward faster than the blink of your beady eye, mate.
Do you believe any frames are missing in this sequence?
Yes. In your alternating GIF there is two missing frames.
In the following GIF centered on Greer's head, I went back a little further to see the position of Greer's torso and as can be easily seen Greer's entire upper half is turned sharply inward towards Kennedy, so your entire premise is based on deception, Greer is not turning his head 180 like Linda Blair but all Greer is doing is a 90 degree turn across his body in the timeframe of about 4 frames
JohnM