Don Eitner
Don Eitner  was the actor who played the Enterprise navigator in the  first season episode . He was also William Shatner's body double as "Evil Kirk" or "Good Kirk" in the episode .

He filmed his scenes for "Charlie X" on Tuesday  and Wednesday  at Desilu Stage 9.

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)

Star Trek appearances

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Performers
TOS performers
Stand-ins