There's no declaring actions in advance of taking them in Scion. People who act on the same tick act simultaneously, which can mean two people attack each other and both die. Both Guard and Aim can be interrupted in favor of another action at any time, but as this game isn't taking place in real time, I'm not going to pause after every tick to ask people taking interruptible actions "do you do something else now?" so please describe the conditions under which you'll interrupt such actions when taking them.