I answered the door OCing and it was a guy and his young daughter selling Girl Scout cookies. I couldn't figure out why they acted so weird until I realized I was carrying
Turn on the porch lights, rack the 12 gauge, crack open the door, strobe them in the face with 135 lumens and demand that they state their business.
If they truly warrant my invitation to enter, I still make them wait outside until I put some pants on.
Many years ago when I lived in town I heard the doorbell ring at a very early hour. I carried an H&K P7 at the time. Had it in my right hand and opened the door slowly with my left. It was a couple police officers. They wanted to talk about some drunk that creamed into the wife's car. I reached over and tucked the gun in a seat cushion as I opened the door for them. Didn't see the need to make anyone more nervous.
People who know us will knock at a certain door, People we don't know, knock at the other. we know right off the bat if the knock is from a known person.