From Keep Your Love On (Danny Silk):
Assertive communicators are unafraid of being powerful and letting other people be powerful within a relationship or a conversation. In fact, they insist on having two powerful people in every conversation. They refuse to give in to the temptation to turn into a T-Rex, a chocolate-covered dragon, or a goat, [he defines these roles in the book] and they confront the other person if they see them slipping into those roles.
A powerful assertive communicator responds to a powerless person with, “What are you going to do about it?”
They respond to an aggressive person with, “I can only talk with you when you decide to be respectful.”
And they respond to a passive-aggressive person with, “We can talk later when you choose to be responsible and tell me what is really going on.”
In other words, they are able to set consistent boundaries around a conversation so that it stays respectful, and they require both participants to equally participate in pursuing the goal of the conversation.