I totally forgot about this. It came up in my game a couple weeks ago and I forgot about it afterward. My player had drawn on an NPC and said "if he goes for his gun, I want to shoot him", and I realized there is nothing to support using an attack as an interrupt. I let him do it, of course, because that's common sense in games with guns and something you could do in SR for as long as I recall. All kinds of tactical turn based games have some kind of "overwatch" concept for attacking out of turn. How about: Overwatch - Minor (I) Enables the use of the major action: Attack (A) after declaring a specific trigger condition.
This is interesting.
I think there are two potential house rule strategies described here, both involving some kind of cost to act later in the round...
1) Reduce initiative strategy - you voluntarily act later in the round, but your initiative is set to the new value of the moment you acted.
2) Commit an action strategy - you can commit to take an action later in the round based on some triggering condition, perhaps at the cost of an extra minor action, but if the trigger does not occur the action is wasted.
Both seem workable to me, but each has their own weirdness.
With option 2 there is really no way to "overwatch" using an action against a person who moves BEFORE you in the initiative order, right? Because your actions refresh at the start of the next round. Maybe that is ok; it certainly increases the value of high initiative scores. And if you make the "overwatch" last until the start of your next TURN, that leads to the weirdness of you sort of having one extra action on your next round. For example, lets say you go on Init 23 and I go on Init 22. If I can Overwatch until the start of my next turn, I could shoot you as you come out of cover on Init 23 with the Overwatch, and then shoot you again on my own turn. That seems too easy to achieve the "double action" that SSDR is warning against.
On the other hand, option 1 can lead to some weird "race to the bottom" situations. Like, I want to move after Bob, so I hold off until Bob's turn. But Bob doesn't want me to act after him, so Bob delays as well. If we both just stare at each other til the end of the round, what happens? Do both of our initiative scores reset to our previous values compared to everyone else in the fight, or are we both now at zero initiative? On one hand, that's seems a bit "realistic" (by some definition); nobody wants to get shot! But on the other hand, it doesn't really lead to pulse-pounding action.