Atheists, you talk to yourselves, right?

[Walking Void]

This right here. Talking to yourself is only a problem when you start believing that it has become a dialogue with supernatural powers.

I thought the problem arose when you answered yourself.

Not unless a second persona is replying. If it is yourself to yourself, You would only be simulating conversation. If a second persona is replying, the experience is real. This can relate into Schizophrenia and Disassociative Identity Disorder.

[Cinjin]

All the time, especially when I'm writing. In fact, I not only talk to myself, I hold entire conversations between characters out loud, to see if they fit.

I do this exact same thing when I'm writing a scene. Sometimes I even move around the room and gesture to make sure it feels like a real conversation.

[Mystical]

I wish I could do that. Typically in order to write a character that doesn't sound like me, I just think of someone I know or have known, and figure out what they'd say.