Having problems with assertiveness at work, I followed some lessons to improve it.
It deals mainly with phone conversations and it looks a lot like what we can see on HZ.

There are 4 stages:

1) The contents (facts-business)
2) Procedure (proposals)
3) Interaction (relationship).
4) Emotional (feelings)

When someone starts a conversation, he talks about facts, argues about them.
What can happen is for the other side to stick to facts and this reassures the caller as that was the purpose of his starting the conversation. The ideal situation.
However, what very often happens, is that the other side becomes personal (using "you") and literally attacks the initiator of the conversation. The problem is that it means that he no longer wants to talk about the topic but about "you". It thus becomes personal.
The whole thing is for the initiator to try to return back to 1) by eventually going through 2) (expressing understanding or leaning towards the criticizer's arguments, slightly.
(I wonder how many phone calls from clients, people here get at work, every day).
I've noticed how quickly some dive into level 3) or 4) (after a factual start).

Examples (samples): 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11,.

OTOH, here's a great example of being able to stick to facts even though he's been attacked on a personal level. And here, it's back to level 2) (procedural) (hat's off to Don,I'd say).

It's not easy to return back to facts after having been cornered to the personal-emotional level.

But it's fascinating to see how some resort to this tactic far more than others.
Maybe, someone going to the emotional-personal level is a sign showing his arguments are weaker than he'd like to admit.

Raoul