Here's a handful of interviews with a few of the stars from various versions of Star Trek about how they got cast for their various roles. The only one from TOS is Shatner...
http://www.cbs.com/shows/star-trek-...ealing-audition-stories-from-star-trek-stars/
http://www.cbs.com/shows/star-trek-...ealing-audition-stories-from-star-trek-stars/