From what I remember she was thinking about quitting the show/franchise early on because she felt the character didn't get enough to do past saying, 'Hey, there's a call for you,' but was talked into staying because of how many girls she was inspiring.
the story they have been saying is it was MLK. He said to her in 67 after the first season that she had to stay. All other roles were black stereotypical roles and positions, but Uhura was showing the future where you could do any role
