If you visit somebody's home in CW to knock on the door, you don't post in the hallway, but in their apartment thread. The main reason for this is that if the character is in the apartment, they may not check the hallway thread for new posts, so will never know your character is at the door. If you post your character knocking on the door in the same thread, although they are outside the apartment, the user will definitely see that you're outside, and you will definitely get a reaction.

Otherwise you could just end up standing outside like a loser.