NPS said:
Showing yourself as "good guy" in defense shooting
I think first we need to define what our goal is. We've just survived fight #1. Now we're about to enter the ring for fight #2. Those two fights are fought completely different. What we don't want to do is survive the fight with the bad guy, only to be shot by the first responders. And that's about the only thing you can do
on your own when it comes to interacting with the police.
"Blue on blue" shootings happen often enough in big city PDs that those departments have gone so far as to develop policies for plain clothes and off-duty officers. Some policies may even include the practice of keeping a jacket closeby which can be quickly put on that has POLICE in large letters across the back. Even with those precautions, it still happens.
Scenes of shootings are usually chaotic. There may be many more people present than just you and the bad guy. If he has friends, or especially family anywhere nearby, expect them to show up. They may even get there before the police officers arrive, flag them down as they pull up, and say things to them that describe you as dangerous to the public.
How you dress matters. Your body language matters. How you appear in your first impression will matter more than anything you say. We all make first impressions about people within the first few seconds of seeing them. The first responders will probably see you before coming within conversation distance. Once inside conversation distance, nothing you say in those first five seconds will have much impact.
Right now just plan ahead and resolve that whatever you intend to say will not be heard over the sirens, the confusion of the scene, and their commands to you. In all likelihood even if they do hear it, they aren't going to make their decisions off of it. "Officers, be careful! He still got his gun," might get their attention. But beyond that, just assume your words will have no impact. Do exactly as you're told. Do not resist a physical takedown. Be helpful in ways that look out for their safety, because in those first five seconds that's all their concerned about.
You want to survive those first five seconds. Think objectively about how you look. Do you look like someone the police come across and arrest every day? What do your clothes say about you? Phoning in your description might work. But it assumes two things - your description will be passed onto the first responders with enough accuracy to identify you; and you're the only one making that call.
We need to find ways to buy ourselves enough time to not get shot immediately, and convince them we are not an immediate threat. That is not the same thing as trying to convince them we were not the bad guy in the fight.
The "shut up and call a lawyer" stuff is good advice. For fight #2. But we need to survive fight #1 first, and that fight doesn't end until all the guns are put away, including the ones the first responders bring with them.