I've always had the impression that most people do not internalize. I get surprized how fast interviewees on TV respond to questions. They can answer, even deaply, without a pause?
I was once (only once) on a raidio talk show with some friends, to plug our sci-fi convention. The host asked me a simple question, and while I was thinking how to answer he went on to the next question to someone else. This happened twice then he stoped calling on me at all.
I ALLWAYS rehearse (in my head) everything I say or write. Hopefully the pause is short enough that people don't notice.
There have been times of stress when this internal dialog came close to the surface. Actuall 2 way dialog. Replaying old conversations and arguments. Sometimes with hand gesters or lip movements. And coworkers noticed.
On the outside, I don't even talk to my cat. (but he talks to me anyway)