Question:

If I quest successfully (for example, my willpower is 3 more than enemies threat) and the active quest still requires 2 points to be completed, is the third point then added to the next quest? Or is this lost?

it is lost. however in locations the excess is carried over. so if you have a location with 5 points remaining, and you quest for 10, the 5 would go onto the quest. if the quest had only 2 remaining the extra 3 would not carry over from the quest though

rich