Interrupt takes place before the action that cuases them. So, they attempt to attack, you can use your interrupt power, which lets you move them away, or attack another target. Similiar to the fighters Combat Superiority feature, if you were to attempt to charge past a fighter, and they hit you with an AoO; the triggering action was the charge, but because you were stopped before it was completed, you lose your action. It takes place immediately before the tirggering event.