As I understand it, the battles my family participates in are somewhat loosely scripted. For one thing, as someone mentioned above, they might say one side wins in the morning battle, and the other side wins in the afternoon. Or they might say they will perform certain major maneuvers which, to appear at all realistic, should have certain responses. Say they might want to have a bayonet charge, they might want to have the artillery play a certain role, have set lines or moving skirmishes. That sort of thing. It really is a cooperative effort in order to pull it off. In all but the biggest events you are using pretty small groups to give the impression of what larger units would have done. Really sucks when you have a few individuals or units who have a rep for refusing to die.
The other issue is that these folks are playing with heavy, pointy things that explode. So you want some conventions for safety reasons. Generally, folks elevate their guns slightly rather than shooting right at opponents, and I believe there is a minimum distance under which you aren’t supposed to shoot at someone.
You can generally tell the more experienced re-enactors, because they tend to die in the shade!
My family all have back stories, tho there is wide variation among re-enactors. One thing is re-enactors are generally quite insistent upon appropriateness. So, for example, my youngest daughter is quite the clotheshorse, so they had to come up with a historically acceptable explanation for why she would have clothes of a certain quality while hanging around with an army unit.
Having a back story also helps if a re-enactor wishes to interact with the public.
The individual units are all historically documented. None of them are just generic “redcoats.” Instead, you’ll have the 55th, or whatever. And members of that unit know why they are wearing each piece of gear they have, what battles their unit was involved in, etc.
In Williamsburg at UTR, they had actors playing Cornwallis and his staff. I hung with them a bit - they were a hoot!