Back in the day, television really worked hard on interesting recurring characters. I'm not talking about Bruce Campbell's Sam on "Burn Notice" or even Neil Flynn's "Janitor" from "Scrubs". I'm talking about those oddballs who were made even odder by the fact that they only popped up every 4 or 5 episodes or so.
Charles Fleischer, who is probably best known as the voice of Roger Rabbit, played Carvelli on "Welcome Back, Kotter", and when he popped up on an episode, it was almost alway assured to be one of my favorites. Carvelli, resplendent in his long trenchcoat, and accompanied by his trusty sidekick, Murray, was usually seen loitering about the halls of McKinley high school, often with something to sell. Whether it be a difficult to obtain object, or some seedy information, Carvelli could be counted upon to furnish it.
Another purveyor of information was colonel Samuel Flagg of MASH. He usually popped up on an episode of the Korean war comedy typically in search of an alleged commie among the ranks of US Army personnel. He made people close their eyes before he left so they couldn't follow, and often called himself "the wind". If anyone was intimidated by this CIA operative, it typically was only Frank Burns, afraid of his own shadow and dumb enough to talk to it. Flagg was a treasure.