In the clip below, I believe that you can see each of the limo occupants simultaneously react to the sound of the first shot.
JFK suddenly drops his right hand from his hair and looks to his left (towards Jackie). Then suddenly returns his head to the crowd on his right (probably reacting to the sound of someone yelling).
John Connally first turns to his right, then quickly turns to his left (towards Nellie), then back to his right.
Jackie suddenly turns her head further to her left, then back towards the front of the limo.
Nellie Connally suddenly turns to her right (towards John Connally).
Agent Roy Kellerman suddenly turns his head to his right, then quickly back towards the front.
Agent Greer (driver) appears to have his gaze towards the camera (Zapruder) and doesn't appear to move his head. He is the exception, perhaps Zapruder (up high on the pedistal) caught his eye.
And last, but not least,
William T. McIntyre (on the left running board of the presidential follow-up car) Hickey suddenly looks to his left and down. He appears to me to be looking at the tires of the limo for a blow-out. (Perhaps he thought the loud noise might have been a tire blowing out.)
Edit: William T. McIntyre is almost hidden behind Clint Hill. However, I believe I see him suddenly duck his head and look around Clint Hill.