I often think I'm like an actor practicing his lines. I run through conversations in my head to see how I sound. Often after hearing it in my head, I make changes or decide it best not to have the actual conversation after all. Why do I do all this? One word. Anxiety. It's fear of saying the wrong thing. Fear of pissng someone off. Fear of ............ You name it. As for people who say they are not thinking about anything. That's bunk. They're thinking. They just may not want to say about what. Or its insignificant and it flies out of their head almost instantly. If you are troubled by the constant making up conversations in your head, I would ask you what you are worried about at that moment. But its all very normal I think.