(written from a Production point of view)
Don Eitner (29 November 1934 – 9 March 2018; age 83) was the actor who played the Enterprise navigator in the Star Trek: The Original Series episode "Charlie X". He was also William Shatner's body double as "Evil Kirk" or "Good Kirk" in the episode "The Enemy Within".
Besides his two Star Trek appearances, Eitner played larger roles in many television series and movies including 12 O'Clock High, The Fugitive, Lost in Space, Desilu's Mission: Impossible and Mannix, Columbo, The Rockford Files, M*A*S*H, Cannon and Dynasty, spanning four decades.
Eitner died on 9 March 2018 at the age of 83. (SAG-AFTRA Magazine, Summer 2018, p. 49)