When you hear the doorbell ring or a knock on your door, safety says, “Do not be in a haste to open the door”. You should first peep through the door lens or peep-hole to see that you recognize the person. If you recognize the person, you may open for them. But, in the event, you do not recognize the person through the peep-hole on the door, you should ask the person for their name, and who they want to see, before you open the door.
In both instances, you should wear a smile on your face as you open the door, regardless of whether you know them or not. Courtesy demands you do just that without hesitation.
You do not want to open the door for a visitor with your face as stiff as a rock. A smile will really cost you nothing but good. Correct?