I think I've just realised from reading these replies, what part of the 'problem' is. It seems that what, from my point of view, is someone who's totally absorbed by enthusiasm with an idea, and to whom nothing but the idea matters, and who has in a sense lost his own sense of self in a state of mind where all he wants to do is gather more information to add to this idea, in order to do which he needs to poke it out of others (in the form of their reactions to the idea), can come across quite differently from the other side.
It can seem as attention-whoring, or great self-confidence or social ease, or indeed, assertiveness.
In effect, it seems like what I need to do with my INFJ friend, is to somehow make her aware that when I'm in that mind space, what she's actually talking to is an incarnated idea, or perhaps a hub around which float ideas that have magnetized to it from the ether, which is presenting itself to her for whittling, adding to, moulding etc, and not a person who is challenging her or throwing down gauntlets or insisting on anything.
What do you think?