I have to say, I often re-run arguments I've had and take them off at a tangent. I also seem to do new ones as well, reacting out loud to comments that people I know either have said or could reasonably be expected to day.
It can actually get me quite stressed and angry. I tend to keep a lid on it when out and about, but I do get caught out from time to time, particularly when stressed. I have also been known to 'pull facial expressions', which have earned a few unfavourable comments from time to time. This is noticeable in the workplace, if I let my thoughts wonder and inadvertently say something.
At home, I consider it reasonable to just be myself, but that can be a mistake too. A 'friend' (read, clingy freeloader after free board and lodging), who was staying gratis in my flat for an unspecified period of time while I was at college often commented on behaviours such as grinding my teeth in my sleep (how would I know I did that?), facial expressions with no apparent external stimuli, tensing up for no apparent reason and either muttering/whispering or saying things to myself.
This 'friend' ensured quite a reputation against me by making sure that everyone else knew about it. No wonder I was considered a 'freak' and a 'weirdo' by the rest of them!
I think one of the correct terms for these things is 'internal dialogue', but with most people, they do not spill out into physical expression.
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"The power of accurate observation is called cynicism by those who have not got it." - George Bernard Shaw (Taken from someone on comp.programming)