Ah. I see we misunderstand each other. What you say is not against what I mean to say. I just happened to use a word that made you think of something else than what I was thinking about. Yes, we might call duets "interaction". But I was referring to when we do formal speech, as to be understood by someone. That's a different sort of interaction. There's no rigid term for it. And yes, solo stuttering is also a thing. That's why I threw in the line about variation. Perhaps see below for my elaborations on the phrasing of it all.