[Kobal2]

By default, archers will "fire closest", pure melee infantry will "attack closest" and hybrid infantry (like Ermor infantry which carry a couple javelins) will "shoot closest" then switch to "attack closest" when the javelins are gone.

H1 and H2 priests will Bless any sacred in range first (including themselves, and even if your God doesn't have magic paths to bless with), then Sermon the rest, then Stay Behind Troops. H3+ will throw Divine Vengeance on themselves first, and Smite instead of SBT. All priests will automatically Banish undead and demons if they get the opportunity. Commanders without any magic will Stay Behind Troops by default I think.

Mages are a complete crapshoot. You can rely on them to cast buff on troops who don't need them, get in a coma from buffing themselves with miscellaneous crap, throwing AoE spells at chaff your own troops would kill without effort (with the friendly fire that comes with that), or to use single-target spells at the closest chaff rather than the big hulking SCs further back.

So, even without you setting orders or shifting positions around, no, your troops won't just sit on their asses with a puzzled look in their eyes. They'll just perform as well as PD and most AI armies, which is not very well at all.