New to Star Trek. Started the original series this month and am almost finished with the second season. To the best of my knowledge, i am not for sure if this question is answered in the series or it has to do with the actors personally or perfessionally. Ive noticed that certain episodes cast different people for Sulu and Chekhov but have not found a definate pattern in their replacements, meaning their not the same replacements every time. Uhura and Scotty are the only co-stars ive seen to be in every episode. Long question short, why is this?