Hello willzp, welcome to DRTV. Most of us hope to never encounter this, however, having a predetermined plan is going to be helpful.
If you called 911, do not hang-up, set the phone down and tend to your immediate threat. Have your wife/girlfriend, etc,.. stay on the phone, and give them a description of you. This is a difficult thing but get the BG on the ground arms spread, if he doesn't comply, and runs, you still can't shoot as your life is not in immediate danger if he is fleeing out the door, and will come back to haunt you.
BE AWARE OF EVERYTHING.
Very "in the moment" depending on the exact scenario. Prepare for them all, and by all means, if you or your family is directly threatened, protect yourself with all means necessary.
"I have an intruder in my home at gunpoint, I am the homeowner in a white T-shirt and boxer shorts, with blonde hair, my name is "XXX"....
When the Police arrive, very SLOWLY place the gun down and keep your hands in plain view. You must be very cognitive of your every move, despite the adrenaline, fear, etc,.. BE COOL!!! It will be hard not to scream THAT"S HIM!! THAT"S HIM!!
Don't do it.
Remain QUIET, the cops are going to consider you a threat until they secure the scene. Let them. They might even handcuff you until it sorts out. Again, STAY COOL!
Rob Pincus has several posts here, as well as many other resources that go into this further. The big thing is, when the cops show up, put the gun the hell down, and keep as calm as possible. The cops are going to take charge as soon as they get there, and will secure the scene first. Breathe and think. Do not volunteer a chat session with the cops. Think, and answer their questions.
BE COOL. Stay on the phone, put the gun down when the cops get there and STAY COOL.