Gary Lockwood starred as Lieutenant Gary Mitchell in the second pilot episode of Star Trek: The Original Series, "Where No Man Has Gone Before." Assigned as helmsman to the USS Enterprise under Captain James T. Kirk (William Shatner), Mitchell's friendly and accessible countenance allowed for a good relationship with his fellow crew members and echoed Kirk's preference for a more casual command style and atmosphere as standard. Lockwood's prior casting in the television series The Lieutenant by the show's creator and producer Gene Roddenberry led directly to Lockwood's role as Gary Mitchell on Roddenberry's next project, Star Trek.
https://screenrant.com/star-trek-tos-gary-mitchell-lockwood-kirk-enemy/