Regardless, though, characters like Sulu, Uhura, and Chekov -- even Chapel and Rand -- were clearly not extras by any definition of the word.
They were not the stars and I would argue were not "main characters," but they were continuing characters who were regularly written into the script with specific dialogue. Not just some random line a director made up on the set. Sometimes it was just the de rigueur stuff like "hailing frequencies open, sir" and sometimes it was even was more meaty stuff, like Sulu in "The Naked Time" or Uhura in "Mirror, Mirror."
Were they stars or main characters? No. But they were clearly regular supporting characters who could, in no reasonable use of the term, be classified as extras.