What is this persona in function speak? How are you certain the idea is correct? What do you measure it's validity by?
EXTPs appear to measure the validity of our Xi feelings/ideas by our Si history of feeling/idea deployment.
When you carry on ideas and conversations in your head-with Ni-does the other person speak as well? It sounds like they involve Te being deployed internally in an Ni intuition? I ask as I think ENFPs do something kinda like this, but backwards wrt to Fi-Si constructs.
It's still Ni/Te,
but the difference is where before understanding had not yet been achieved, now one has achieved understanding, to the point that we're rather sure that we're not incorrect. Also to clarify, I'm not talking about stating half-assed opinions for the sake of argument, but rather such a full and deep understanding that when one says something, others are surprised at the depth of knowledge in play, and are either amazed at the accuracy and insight or are intimidated by it. Like watching a figure skater at the Olympics, what you don't see is all the days/weeks/months of effort that went into being able to
do that, you don't see all the effort, the raw experience, skill and training, that the INTJ put into his statement.
Moreover, the INTJ is stating that which is
obvious to him. He did all the hard work a long while ago, when you weren't looking, and now it is time to act, bringing that "J" into play. The idea was tested over and over and over, in his head and in real world observations, and then was carefully stored as an Ni functional pattern (functional is in the kind of idea, not Jungian function). That "in his head" testing is sort of the "inner conversation" thing you mention, but more accurately it's running multiple scenarios, in a "quality assurance" kind of way. We keep trying out ideas and discarding the ones that "don't work." Again, it's all functional ... it isn't about true/false, but workable/unworkable. Does the plan get me from point A to point B, for example. The few ideas that get past this test, are tested against the real world.
So by the time he's
talking about it, the idea or plan or whatever has already existed a long while, except it's stored in Ni, not Si (as would be the case for ISTJ). When he explains it, he doesn't have to figure out how to explain it, because it's already been tossed back and forth between Ni and Te for a while. He's explained it to himself many times already, and edited it so that is clear and understandable, especially if it requires explaining to others.
So it isn't a different Jungian function rearing its head, it's the same ones as before, only now it's in play. Perhaps your observations w/r to Se might apply, that once it is time to present the idea to others, Se gives the INTJ a great deal of presence that he doesn't normally have. He is now about being active and doing things and executing the plan, not discussing or debating. I imagine that many others, for whom the plan or issue might appear to be very new, will resent the manner of presentation and find it overbearing, feeling like they have little say, because the INTJ has already answered all of their objections in his head, long ago (he thought of the objections first!). Hence, the arrogance.