The breakthrough in the design of Zero Dark came from the simple realization that I already had the best possible design tool available in Horizon Wars.
There are a number of design principles at work in Horizon Wars that are distinctive to my approach to game design - the generic nature of the rules, the ability to build your own units, the stripped down approach to stats - but whilst they were distinctive and would, necessarily, repeat in Zero Dark, they weren't unique.
What was unique was the shooting mechanics.
If you've not previously played Horizon Wars, shooting works by measuring the range, adding a few modifiers and rolling a number of d12s equal to your unit's "Firepower" stat. You then create groups or clusters of dice from the result that equal or exceed the target number. For example, if your target is 17" away, with Armour 2 in +1 cover, your target is 20. If you roll five d12s and get 3, 7, 8, 8 and 12, you have one hit (because 8+12 is 20, but 3+7+8 is only 18). If the same target was only 5" away, you would have four hits.
It sounds weird when you hear it, but it is very intuitive once you get started.
In Zero Dark, I took this simple mechanic and turned it up to 11. Basically, I applied it to every test you could imagine. So it's not just shooting, but making cautious moves, trying to heal a wounded comrade, trying to hack the enemy datanet...
But what I realized when playtesting this mechanic was that I was ending up with a lot of surplus successes. It feels great to roll four or five successes on a test, but when you only need one... you get a bit of a feeling of letdown. At first, I tried "banking" successes to use against future tests. But quickly discovered that this just encouraged you to take tests for irrelevant, easy stuff and store up successes to auto-pass the difficult tests. No fun at all! So I quickly hit on the idea of "bonus actions" - a limited stock of extra things you can do to spend your surplus successes.
There are a number of options, including moving, shooting and taking a support token (a relic of the "banked success" concept). This gives a hero the chance to snap off a shot then duck behind cover, for example (but a risk that they'll not have enough successes and get caught in the open). But even then, I found I had surplus successes when there just wasn't something you wanted to do. So, quickly, bonus actions were made available to the whole team to use, which also addressed another problem - that you could only activate one character at a time.
With bonus actions, multiple heroes can act at once.
But there are strict limits. The main one is that, in any one activation, each bonus action can be performed only once.
A good way to illustrate how it works is with another important mechanic: the cautious move.
Movement is either cautious, normal or urgent. Normal movement is twice a hero's Mobility. Urgent movement is three times, but comes at a significant cost (you use it to dash into cover across exposed areas, or to get to the exfil point at the end of the game). Cautious movement, meanwhile, is only up to the hero's Mobility, but you make a test (as described above) with the target going up the further you move and the more enemies are nearby.
If you succeed, your first success cancels the next Control Deck flip (which can be really handy!), and surplus successes can then be spent on bonus actions.
So if a hero with Mobility 4 and Discipline 3 cautious moves 4" with two enemies within 12" and in LOS, the target is 12. You roll 3 d12 and get 4, 8, 12, giving two successes. BUT WAIT!
A 12 is a "lucky 12" and always confers +1 success. So you get three successes.
The first success cancels the Control Deck flip (slowing the Red Force and removing their special abilities). You can then use the second success to move again, up to the hero's Mobility, for a total of 8" (just as if you'd done a normal move) and you still have one bonus action to use to snap off a shot at that enemy on the corner.
Better still, bonus actions can generate more bonus actions. So if that shot kills its target and has surplus successes, you've got more bonus actions to use. BUT WAIT!
You've already moved and shot in this activation, so your options are getting limited. You can take a support token, drop prone or - if you're next to an obstacle - jump up/down/across it. If you've already got a support token, don't want to drop prone and have nothing to jump up/down/across, what can you do?
Don't forget you can share your bonus actions with other characters. So in this case, another character can get a bonus action to take a support token, jump or go prone.
As with most games, there are special abilities can play with these mechanics - such as ones that let you spend multiple bonus actions on shooting, or on movement - but these ideas sit at the heart of Zero Dark.