The large skirmishes I've played didn't have roles, so I can't be much help regarding roles. But the organizers, Combat Enterprises, are cool guys and I'm sure they brief you properly. The CE guys have put on many fun /successful multi day events. Just ask alot of questions before things start. Try and go on Friday,
I have played on the Agua Dulce film ranch a few times though. Take extra socks and keep them dry in a ziplock. You're not neccessarily going to get your feet wet but you will sweat. Hope you have good boots. the under $80.00 ones will just give you blisters. Camelback or other non "sloshing" hydration pack and make sure it's full and you drink alot. You can even fill with gatorade so you get some salt/electorlytes in your system.