The easiest thing I can tell you, while it is still a very hard thing to do, is to remain as bored as possible with the narcissist that you are dealing with.
Once you leave, if you have children, you will have to deal with the narcissist from time to time. Remember, if they are being nice, they want something and if they are talking, there is a good chance it is some distorted view of the truth. Use their last name and in your mind say to yourself “I refuse to enter the _(last name here)_world!” I used to say this when married, out loud. They want to engage in conflict. They live for it so put yourself in the neutral zone. Think beige or gray. I remain poker-faced even when on the phone so that the narcissist knows just how bored I am of his games and tactics. I have even said, “Good Lord, now that I have a real man in my life who knows how to communicate, talking to you is like watching laundry dry on the line. Your games are so boring! When you want to have a real conversation about our children, call me back. Until then, I just can’t be bothered.” Then I say goodbye and hang up.
The key is no emotion. No taking the bait no matter what the narcissist says. I’ll be honest, the one area I am still working on is when he involves our children or hurts them in some way emotionally. It takes all I have not to rip into him and I have far from perfected my poker face in that particular game but I’m working on it.
Remember, he likes chaos and disorder. He likes you to be off kilter. Always think beige or gray when dealing with him. Be as neutral and droll in your responses as possible. Have the conversation that is needed and get out and if he can’t have it, tell him when he gets it together, you can talk then. It will empower you and you won’t have a chance to fall victim to his games.