Neddy Merrill at The Edge of the American West (a great American history blog, if you're interested and aren't following it already) shares an interesting anecdote about Nichelle Nichols, who played Lt. Uhura on the original Star Trek:
Apparently Nichols almost left the show because she had a Broadway offer, until a chance encounter with a fan changed her mind.
That fan was Martin Luther King Jr. Nichols recalls their conversation:
"One of the organizers came up to me and said that there was someone who wants to meet you; and he says that he’s you’re best, biggest fan and I’m thinking it’s a Trekkie! [laughs] and so I said certainly and I got up and turned around and maybe 10 or 15 feet coming towards me I see Dr. Martin Luther King and I remember thinking whoever that little fan is, he’s going to have to wait, because here’s Dr. King, who walks straight up to me with this big, magnificent smile on his face and says, “I’m the fan!” because I’m sort of looking around for someone else, and he says, “I am your best fan, I am your biggest fan!” and I… I was at a loss for words, and if you know me, I am never at a loss for words.
…and so I told him I would be leaving the show, because; and that was as far as he let me go, and he said, “STOP! You cannot! You cannot leave this show! Do you not understand what you are doing?! You are the first non-stereotypical role in television! Of intelligence, and of a woman and a woman of color?! That you are playing a role that is not about your color! That this role could be played by anyone? This is not a black role. This is not a female role! A blue eyed blond or a pointed ear green person could take this role!” And I am looking at him and looking at him and buzzing, and he said, “Nichelle, for the first time, not only our little children and people can look on and see themselves, but people who don’t look like us, people who don’t look like us, from all over the world, for the first time, the first time on television, they can see us, as we should be!"