When I am alone - it could be walking down the street, it could be at the bus stop, it could be in the shower - I too play out conversations and scenarios in my head. I am very self concsious of this and sometimes get scared I might have been mouthing out the conversations while in public. Eeek.

As for being on autopilot. That is normal. I'm sure most people can think of a time they went somewhere and don't remember most of the trip. I have ended up places and been mortified at the thought I might have gone through a red light, etc. This hasn't happened often but it has - and it is normal. It is called subconscious competence.