In the book it gives an example a pc pulling a gun on another pc and the pc being attacked acts under fire to avoid while the attacker interferes. But I wonder if this is the only way it should be done or not.
I don't think there's any single way to resolve this question, I think you always have to make some MC decisions based on the current fiction and what the players say they are doing.
Examples:
1)Player 1: "I'm drawing my gun..."
Player 2: "And *I* am getting the fuck out of here!"
MC: "Player 1, you doing anything to stop that?"
Player 1: "Shit yeah, I'm unloading on her!"
That would be Player 2
acting under fire to get out of there and Player 1
interfering with their shooting.
2) Player 1: "I'm pulling my gun and blasting her, 'Die bitch!"
MC: "Player 2, you doing anything to stop that?"
Player 2: "Yeah, I'm bobbing and weaving and diving for cover!"
That would be Player 1
going aggro and Player 2
interfering with their evasions.
3) Player 1: "I'm shooting from my hip, blasting her before she can react!"
Player 2: "I said I was under coming here suspicious of your motives and my Read a Sitch proved me right, I'm ducking!"
That would be Player 1
acting under fire to get the shot off, maybe even with AP, and Player 2
interfering with, perhaps, a +1 from a
read a sitch. If Player 2 hadn't read the sitch, they might not even get a chance to interfere here!
4) Player 1: "Fuck it, I'm shooting her."
Player 2: "And I'm shooting back!"
MC:
Trade harm for harm! (OUCH!)
5) Player 1: "Fuck it, I'm shooting her, you're going to die crying like the coward bitch you are!"
Player 2: "And I'm taking you down with me, baby! BLAM! BLAM!"
I'd say this is Player 1
seizing by force, trying to seize the other player's
reputation perhaps, with Player 2 fighting back directly as
interfering.
Does that help?
I'll see if I can think up any stranger combinations...
(See you Sunday!)