I talk to myself. Or, I talk to others when alone. At times, the dialogue, or at least my part in it, is audible, and at others quite an interior event. The embarrassment of being caught in this most inappropriate behaviour will generally suppress it, but a period of isolation can bring it to the surface again.
If I was staying to look after him, I could hear my father, as he lay awake in the morning or after a nap, angrily engaging with the demons irrupting to consciousness from long halls of painful memories. He was old, he was deaf, and was no doubt unaware of just how his interior struggles infiltrated the quiet house. In any case, he would probably not have been concerned.
I'm not so old as to be unconcerned. Except in extremis, I generally remember to keep the volume down, but the monologues go on. There may be a vast assembly of dramatis personae, but none of them get to say much. Others with a finely tuned awareness of the motors of human behaviour may well be able to design and rehearse very practical conversations in the same arena; not me, for worse or better.
Many of these are almost entirely silent. I say almost. Sometimes I realise that the interior voice is lisping. I notice because of the fullness and tension I feel in the top of the throat and up under the jaw to the tongue. I cannot get my tongue, in silence, around the words properly: I am thinking with a lisp.
Not simply thinking, of course. Along with the imagined words must be the slightest of muscular rehearsals; far too small to be audible, but enough to suffer from some temporary dislocation which feeds back to my inner ear as a lisp. So idea, and the inchoate will to express, precedes language, which serves it, better or worse.