At the big fight outside the fortress with the marilith:
The Evil Cultist Archers readied actions to shoot if one of my people cast a spell. One of my people cast a spell, and they took a full attack worth of shots at the spellcaster. Only problem is, you can only ready a standard action, not a full-round action, which means they should only get 1 shot against the spellcaster.
https://www.d20srd.org/srd/combat/specialInitiativeActions.htm