Is it normal i have conversations even when alone?

Ok. So I know talking to yourself isn't really that weird, for instance people would normally mutter these things like, "Oh, gotta remind myself to get milk", or "God dammit Susan you twat!". I've done this all my life, but recently it has escalated to me having full blown conversations to myself or my room. At one point I was lying on my floor and literally talking to thin air. It went something like this:

"Do you think aliens exist? No I don't think they exist. What about you? You twat of course they exist!"

I've also had conversations with various characters I make up or draw as myself, answering out loud. I also cross the barrier between thinking up a conversation. I'll be thinking it then randomly burst out and carrying on the idea. It is also not unusual that I speak to my various posters. This is worrying me constantly and I am quite frankly scared for my sanity at this rate. Can someone tell me if this is normal or not?