Nerys Myk wrote:
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.
Nichelle Nichols was a day player. Doohan, Takei, Koenig, and (for the first 13) Grace Lee Whitney all had long-term contracts -- just not for as much money, episodes, and days (and for residuals at scale, for the most part) as Shatner and Nimoy had.