It could be a special type of attack scaling with companion's attitude, but taking place immediately after player's action.
Example:
Let's imagine that we're playing with our main character + two companions. We've got a mage with lightinig spells and some sort of cleric(I'm not sure if such characters will exist, but it's just an example). Mage likes us, cleric not. The fight stars and we throw a barrel of water into an enemy. The enemy gets wet and immediately after it, our mage cast lighting bolt electrocuting him. It costs no action points, nor mana - it's a reward for beeing frendly with the mage. On the other hand we've the cleric. On the next turn some huge monster appears and deals a lot of damage to our character. If the healer had liked us, he would have cast heal spell (as an action of opportunity). But he doesn't, so we have to heal on our own.
Personally, I think that it would make relationships with companions more gameplay-affecting and thus more rewarding.
How do you feel about it?
