I get the Overwatch mechanism.
Except that it is only for Movement (changing zones).
As written it does not work if it is free movement in the same zone ("If you reach for that grenade on the table over there, I shoot").
Here is an example from my players:
• PC-T. does not want PC-G. to kill the NPC that just shot one of the other PCs.
So, in Phase 2, the PCs see that the NPC is not going to shoot again (not declaring Ranged Combat).
And T. wants to: "G., if you move to hit the NPC with your axe, I shoot you" (shooting when G. starts the action, if he raises his axe etc.--before he hits).
Game-wise that cannot work : T. must declare he shoots G. and then the roll is resolved, before Phase 3 where G. actually declares he will attack the NPC.
And I had two or three in the first rounds of my first Brawl.
Someone stating they will heal someone, and another player saying that their action will be to shoot if there is attempted healing.
My feeling is that I will run it as Overwatch:
• you declare your Action in that Action's Phase, but can put a condition to using that action (for Overwatch it is if someone Moves into an Area ; here it can be anything: if John starts the Jeep's engine, if Bill knife's the dog...)
• you cannot change your Action, and if your Action's trigger does not occur, you 'lose' your action for the round (like Overwatch)
• but if the trigger occurs, then you resolve your rolls in the of the Phase in which it occurs
Quite similar to Overwatch, except that it has a broader trigger, and that trigger can call for a roll in other Phases than Phase 4-Movement.
• since the goal is often to prevent something from being done, my feeling is that I would probably allow that if you have changed phases.
If you declare Ranged Combat and then act against Close Combat, or anything from Phase 3 and more ; if you declare Close Combat and act against anything Phase 4 and more
But if the trigger is in the same Phase you can act, but not before the other person, so not prevent
Playing in France, and online