For me there’s two separate participants, a ‘talker’ and a ‘listener’. My mind identifies more with the talker, because that’s the one that has agency. Since there are two participants, both of which are me, I talk in 1st person plural (‘we’ve got to do …’, 'we thought about this earlier’). I stopped being afraid of being alone after I started having an internal dialogue around the age of 11, since having a second participant in the conversation meant I was always in company.
Edit: Wow, looks like there’s a lot more diversity in this than I was expecting
Sometimes the monologue is so loud I end up accidentally vocalising (whispering) it. I think it might be partially caused by the fact I have ADHD and a monologue like this is a way to keep my brain stimulated (thought wise, but also socially) when there’s no input from the outside.