Characters of Convenience
I just watched "Recovery," a fifth-season episode about Japanese internment camps.
It illustrated a technique I call "characters of convenience," where the writers suddenly introduce a never-before-seen staff member because of a plot necessity.
Of course, every episode has guest stars, because the reporters meet new people when pursuing stories. But it's a stretch of credibility when a reporter pops out of nowhere (in this case a Japanese staffer) and everyone treats him/her like an old-timer. Lynne Moody only shows up when we have a black-related story. Others arrive when a reporter-related story involves a problem too serious or life-changing to be attached to one of the regulars. And a whole new staff bloomed out of nowhere for the episode about the "night side" (see my comment on that episode).
Anyone else have examples of "characters of convenience"? (This was also common on THE PAPER CHASE, which ran around the same time as LOU GRANT. New students would appear among the regulars in class...people who had never been there before and would never be seen again after one episode.)