It helps – even without training – to have put some thought into this. Because experts in “actual” security (including those who investigate the aftermath of these types of “active shooter” situations) have provided lessons that can be learned, if they will be, by people who don’t have actual training.
The best advice that I’ve seen is a sort of simple hierarchy of elements that nearly anyone could apply in some such scenario – depending on your awareness and the actual situation around you.
First, don’t freeze. Don’t imagine that “it’s nothing”, or “it doesn’t involve me” or “it’s not what I think it is”. Apparently, it is the air of disbelief that people adopt that gets most of them killed after the initial surprise attack. See, hear and comprehend what is going on around you and resolve to do something – probably one of the next three steps:
Escape / Evade – as the writer puts it very succinctly in the link above, “distance works against a shooter and for a target”, so if you can put distance between you and the shooter, do that as quickly and effectively as you can. Get out of the kill zone.
Barricade – if you can’t escape, for whatever reason, then get behind a solid door, walls, whatever masses are available, and make it difficult or impossible for the shooter to get into your space. You would also want to be as quiet as possible sometimes – it would always depend on circumstance – so that the shooter wouldn’t be attracted to make the attempt to breach your barricade.
Fight – when the other options are gone, then you do what you can to overpower, disarm, disable or kill the attacker.
I’ve now internalized enough of this – just from reading and watching a couple of videos – that when I hear “actual firecrackers” (and I know that’s what they are), I’m already thinking about where I should be going or what I should be doing “if this were an actual attack”. It’s not a lot of practice, but just like a fire drill, I don’t think it takes “a lot” of practice to be able to internalize “what should be my reaction when I see or hear X?” And then start to practice that.