This is a fascinating thread, especially the satirical and religious comments.

Since we have two hemispheres of mind/brain, talking to ones 'other self' or talking to themselves may be an evolutionary advantage of reflection to clarify thoughts and actions. The "friend" some talk to is obviously themself or an aspect of themself.

Talking to oneself is not a schizophrenic symptom, that usually involves being talked to. People who talk to themselves is usually an organizational and decision making process.

Some autistic children who were deprived of speech until the age of 5 or 6 became accustomed to having an internal dialog that eventually became an out loud dialog when they finally learned to speak aloud.
They are the worst liars in the world since all you need do is listen in.

The culturally common example of the old scientist talking to himself is accepted as a normal idiosyncrasy.