Patrick Stewart, Brent Spiner, Rene Auberjonois, Robert Picardo, and Anthony Montgomery all appeared in the Frasier. And it goes deeper than that. This post barely scratches the surface.
Errrrr... It's a little known fact that the, uh, first NPC's were invented by the Romans. Ya see, in the days of the gladiators, they would, uh, place random people in the colluseum to yell random stuff at the gladiators while they were fighting for their lives. The NPC wasn't technically part of the match and you weren't allowed to kill them, even if you really wanted to.