Re: Casting Confusion
The character of Chekov wasn't around for the first season so various actors sat in the navigator's seat in that season, a few for more than one episode. Other actors/characters also filled the helm station when George Takei wasn't available or needed. Uhura/Nichelle Nichols was absent a few times as well. All were day players and so they didn't really long term contracts and worked on a day to day basis.
The boring one, the one with Khan, the one where Spock returns, the one with whales, the dumb one, the last one, the one with Kirk, the one with the Borg, the stupid one, the bad one, the new one, the other one with Khan.