Does the target of the attack have to be character whose turn you are interrupting or can it be any character? The spirit of the card seems to be the target is whose turn it is, but the card text doesn't seem to specifically require it.
It's a single attack on any one character in the game(player's or npc's) on any character's turn(player's or npc's) EXCEPT your own.
Example: 4 player game with 2 npc's. You're player 2 and your turn just ended so your ability card is obviously refreshed. The npc goes next. During the npc's turn, you may attack any one player that is adjacent to you. If you choose not to at this time(strategy or simply because no one is adjacent), you can wait until it's player 3's turn, npc number 2's turn, player 4's turn, or player 1's turn.
But just to be clear, you can only attack once per round. It has to come back to your turn for the card to be refreshed to be ready to use again. So if you decide to attack on npc 1's turn, you cannot attack again until after the end of your turn when your ability card will again be refreshed.
Hope this helps!