Mark Lenard, best known for his role as Sarek, Spock's father, was the first actor to play a member of all three of the major alien races: Vulcan, Romulan (in the episode "Balance of Terror"), and Klingon (the commander of the Klingon attack group at the beginning of Star Trek: The Motion Picture (1979)).
Martin Landau was originally offered the role of Spock, but declined. Later, Leonard Nimoy, who did accept the part, took over the role of the disguise-expert on "Mission: Impossible" when Landau left that show. Landau later headed his own sci-fi series, "Space: 1999" (1975).
James Doohan ("Scotty") lost his right middle finger during WWII. Most of his scenes are shot to hide it. In scenes with a close-up view of Scotty's hands (operating the transporter for example) someone else's hands were always used. However, it is noticeable on a few rare occasions. The most obvious example is "The Trouble With Tribbles" when Scotty comes into the Rec Room with his arms filled with Tribbles. Take a close look at his hands. You can clearly see he's missing a finger.
Walter Koenig, who wrote an episode of Star Trek: The Animated Series entitled "The Infinite Vulcan," became the first Star Trek actor to ever write a Star Trek story. Over the following decades, many Trek actors would write films, novels and comic books based upon "Star Trek", and many more would direct TV episodes and movies.