I remember working rural territory when I was a kid.
Once we drove down one long, winding, rutted dirt road for what seemed like forever, and when we finally got to the house at the end, an overtly unfriendly, raspy-voiced woman came out to meet us, asked what we were doing there, and then said she was surprised we made it that far without getting shot at. We later found out we were out in the area where the local dope-growers lived.
Another time, I went to a small camp trailer with a wooden box for a front step, and when the door opened I was greeted by a long-haired, shirtless man with torn, faded jeans and bloodshot eyes, and a very strange-smelling smoke wafting out around him. I did my introduction, and he answered, "Listen, sis. When you've been to Viet Nam and back you either go one way or you go the other. I accepted Jesus Christ." The Reasoning Book not having been printed yet, I was somewhat at a loss and said, "Ok". He smiled and closed the door.
Those were my two most memorable "Knock Knock" experiences, I guess.