Of course, we did occasionally see people other than the regular cast actually sit in the captain's chair. Or we did at least once anyway, that being Lieutenant Desalle in "Catspaw". Maybe he was part of another shift? "Catspaw" being a Halloween episode, maybe it was even supposed to be taking place at night-time?
(Of course, DeSalle was the assistant chief engineer, so maybe he was just in the pecking order behind Kirk, Spock, Scotty and Sulu.)