There’s been mention of this, but for those not aware:

Special Agent John D. Ready, primary protection for President Kennedy, stood on the right front running board of the Secret Service follow-up car.  His reports stated he stepped off, intending to go toward the President.  “But he was immediately called back by Special Agent Emory P. Roberts...” (Warren Report, p. 51.)  Their SS reports do not say exactly when this occurred.  Neither was interviewed by the Warren Commission.

However, Ready can actually be seen briefly at the extreme right of the Orville Nix film.  Nix’s camera filmed at 18.5 frames per second.  At 1.3 seconds after the fatal head shot, he is still standing upright.  At 1.4 seconds he has begun to step down to the street.  He ends his downward movement about 1.6 seconds (30 frames) after the head shot.  He is then panned out of frame.

 

 

Ready can be seen in this slo-mo of the Nix film from 0:44 through 1:02: https://www.youtube.com/watch?v=OcXjpBFRS5A.