You first declare the attack, if your opponent says the attack is legal your character will exhaust and gain the attacker characteristic. So, if for instance you say I declare and attack on Dr. Doom, at this point your character is still ready. Your opponent at this point can chain effects to you declaring the attack. So, your opponent now decides to play mystical paralysis exhausing Dr. Doom to exhaust your proposed attacker, your proposed attacker will exaust and therefore making your attack illegal so it doesn't happen. If your opponent does nothing then your character will exhaust and the attack starts. At this point you and your opponent can then chain more effects if you or him have any. For instance you opponent can try to bounce the attack by playing acrobatic dodge.
