The companions with "Attacker" classes in my party use skills themselves without having any behaviour set. Both slots are set to "No Behaviour Set" for all companions. A.I. is turned on and auto-attack is set to "Defensive" for some, and "Defend Self" for others.
It doesn't seem to be a problem with spellcasters like Aloth or Xoti. (At least I haven't seen them use any spells unasked)
Maia just throws out all her Ranger and Rogue attacks and is out of resources really quickly, despite no behaviour being set.