It is fairly well known that Leonard Nimmoy came to Gene Roddenberry's attention when he appeared in an episode of The Lieutenant, a TV show Roddenberry produced in the years before Star Trek. That episode was on GET-TV over the weekend. What was surprising was another guest star. Nimmoy plays a movie producer, and this producer has an assistant. The assistant is played by Majel Barrett.
Another familiar face on The Lieutenant this weekend: Walter Koenig. I wonder if I look up the cast of Star Trek on IMDB if I will find that most or all of them will have appeared on it.
This just in:
I looked up the show. I didn't know that the title character (played by Gary Lockwood) has an interesting middle name: Tiberius (although it was supposed to be "just a coincidence" since David Gerrold gave it to Kirk). Also, Nichelle Nichols appeared in an episode that never aired. And I knew I'd seen Gary Lockwood someplace: the second Star Trek pilot "Where No Man Had Gone Before": he played Lieutenant Commander Gary Mitchell.
That's good Trek!
Nimoy's first appreance in a Roddenbury script was even earlier: In a TV series (recently released on DVD) called "West Point", an anthology series about West Point Cadets. Nimoy played a Cadet in a episode scripted by Roddenbury.