Along with TUs, a unit's Reaction rating is one of the most vital resources a unit can have in combat. Reaction influences precedence - who gets priority to make their move. It is used offensively and defensively. The side that's currently issuing orders (the attacker) uses reactions to prevent opportunity shots while the waiting side (the defender) uses reactions to take shots of opportunity.
When two opposing units are within range of each other, the Reaction levels of both units are compared. The unit with the highest Reactions gets to make the move. If the defender is not aware of the attacker, no check is made. However, as soon as the defender is made aware of the attacker, a check is made and will then perform an attack of opportunity if the conditions for a reaction shot are satisfied. Note that if an attacker with lower reactions is exposed to a defender with high reaction due to the actions of another unit (such as shooting away a wall), the check is not made and the exposed attacker is not fired upon. However, the moment the exposed attacker moves, the defender will be able to obtain an attack of opportunity given the right conditions.
What is not common knowledge is how the overall reaction level of any given unit is also influenced by the percentage of the unit's remaining TUs! So say an alien has 100 reactions but only has 60% of its remaining TUs. Its actual Reaction level is really 60, not 100. If it takes a Snap Shot, which costs 30% for any plasma weapon, the remaining TU percentage will drop to 30%, and so will the Reaction level. For more information, see Reaction fire formula. --JellyfishGreen 10:51, 25 Apr 2005 (BST)
With this in mind, one could say that for the best opportunity attacks, end your turn with full TUs, wielding weapons with low snapshot costs. The Pistol, Laser Pistol, and Laser Rifle are by far the best weapons in the X-COM arsenal for attacks of opportunity. Still, these weapons (especially the standard pistol) may not be enough to take down e.g. Mutons. Adapt to your situation.
For melee attacks, it is often best to use a mind probe to examine the stats of any alien that you wish to approach. By getting an overall impression of the alien's remaining TUs and its reaction level, you can get a good idea of how safe the alien will be to approach.
New recruits will always begin with a value between 30 and 60.
Reaction points are awarded (at end of combat) depending on how many times you took reaction shots. Missed reaction shots do count - all that matters is that you reacted. (Misses don't count for Firing Accuracy increases, though.)
You can be awarded up to an average of 4 skill points (range 2-6) per combat mission if you make at least 11 reaction shots. The point award does not depend on your current skill level, only on the number of reactions. For more particulars on skill-point increases, see Experience.
Note: Although there are several area-effect weapons that can be fired with a reaction shot (stun bomb, auto and heavy cannon, rocket launcher), each reaction shot only gives you 1 point of reaction experience, even if the shot impacts numerous aliens. From the game's perspective, you were only reacting versus one particular alien. However, multiple hits from one reaction shot will give you multiple experience points toward firing accuracy skill increase.
For the record: Occasionally, you get more reaction experience points (Unitref) than rounds fired. It can be readily seen when numerous soldiers react to enemies, such as with a firing squad. Each time an alien goes down (dead or unconscious), one of your soldiers is likely to gain an extra point. You may even see 15 reaction points after firing a clip of 12 bullets, although this is rare. What causes the extra points? Possibly reactions are "queued" when reaction ability is checked, but then the game doesn't make sure that the target is still viable until the next queued reaction comes up. You won't get an extra point every time an alien goes down, even with firing squads, though. This is probably because the last queued reaction on "your" side (before the enemy acts again) put the alien down. All in all, the more soldiers you have with similar reaction levels (TUs x Reactions, so they all all queue up at the same time) and with shorter snapshot times (puts more shots in the queue), the more likely you are to get these extra points. And the max possible extra points equals the number of targets. Still, unless you're peeking at Unitref, it's practically impossible to know when it happens. So sometimes you get a bonus to your reactions, even if you didn't know it.
Tip: If some soldiers have high reaction and some have low, the high ones are liable to get off a ton of shots before the low ones, and kill the target. Thus, the high ones get even higher, distancing themselves even more from the low ones - a vicious circle. If you want to build up low Reaction skills, try to keep "reaction stealing" in mind and work around it. If you have the funds, most players recommend recruits with at least 40 Reaction, if not 50 - there seems to be a definite notch where low Reaction soldiers get left behind. Also see the links below for more tips on training skills.
[ NKF: Alternately, arm the high reaction soldiers with slower-firing snapshot weapons (heavy laser/cannon, auto-cannon or rocket launcher, for example) and arm the low reaction soldiers with faster firing snapshot weapons (pistol, laser pistol). This evens things out a bit, and although the higher reaction soldier will fire first, the higher TU cost lowers the reaction level considerably. And the lower TU drop for the faster pistols means the soldiers with lower reactions will not drop in reaction level as quickly. You've just got to remember that no matter how high your reactions are, they're meaningless if you don't have any time units.
It's always a lot better to train reactions as a group, with some high reaction soldiers to soften up the enemies a bit. The more the merrier, as they say. Also, the sight of 8 - 10 soldiers with laser pistols unleashing a seemingly endless amount of bolts of light at a couple of aliens leaving a UFO is sight any player should behold, at least once.
Some food for thought regarding the reaction skill. Does the reaction counter increase with an unloaded weapon? Or does it happen in the scenario you mention, when there's insufficient TUs to fire, but your reaction level is greater than the alien's.
Here's one related question I'd like to see answered sometime: What actions cause an attack of opportunity to be triggered? I ask this because not all actions that use TUs will trigger it immediately, such as muddling around in the inventory screen.]
[ MTR: [snipped - see below]
Roger that re: what triggers opportunity fire. I wonder, too. It's also 100% clear that doing a psi doesn't cause reaction fire - and it takes lots of TUs. I have also never seen an alien fire on me when I was only turning, although I have seen cases where I fired on aliens, when they only turned. I've been trying to keep an eye on this for several weeks now, and haven't noticed any exceptions. This makes me heavily wonder whether it is tied to energy usage, not TUs, per se. Remember that most aliens have U=0 and use energy even when they turn, but soldiers have U=1, and don't. See what I mean?
Another thought to throw into the pipe: I wrote this second paragraph about personal lighting. Something all us vet players know. But since then I've realized that movement that doesn't use energy (elevators) does not trigger "backfield" personal lighting. And, just did more testing - in fact, energy usage is needed to trigger any check of aliens' personal lighting. Examples:
- You MC some aliens on a regular turn; people move; aliens are lit, as usual. Start of next turn: Still lit, although no longer MC'd. Move any of your soldiers so that they use energy - the formerly-MC'd aliens go dark.
- Same situation: MC some aliens, they light up, end turn; start of next turn, formerly MC'd aliens still lit. Move one of your soldiers up or down on an elevator: They stay lit. Move her off elevator: They go dark.
- Same holds for formely MC'd aliens or aliens you just MC'd, and moving them. Whether they are 'supposed' to light up or go dark, no difference... Movement up or down on elevator: no change (no personal lighting check). Move off elevator: aliens change status.
- Whether they should be lighting up or going dark, energy usage by you or them is needed to trigger the alien lighting check and make them be "correct".
- But wait! Beginning of turn: formerly MC'd alien called "A" is still lit. MC some other alien called "B". Now move B on the elevator: A is still lit. No problem. But turn B so he uses energy: A is still lit. Move B off elevator: A goes dark. ARG! B used energy, but A did not change status...
These are interesting findings in and of themself, especially in those dark windy alien base "gardens" - a.k.a. don't move anybody and you'll see formerly MC'd aliens better! Anyway, clearly, alien personal lighting checks interact with energy use somehow, not movement per se. Maybe the equation behind the scenes has U=1 for everybody, and doesn't check the unit's U, per se?? But then that would counter my obs. that I can't remember aliens every reaction firing on me when I was only turning. Has anyone ever seen aliens fire on them when all you did was turn?
So there's at least one "movement" thing that we now realize is actually related to energy use, not movement per se. See, they are so very closely related, that the programmers may have used that as a simple way to differentiate all the many possible actions, vs. ones that really "count" toward certain things. While I'm not at all certain about this, one quick check would be to hack Unitref to no energy usage... then go waltzing out in front of aliens that are loaded for bear and see if you make it across the field... reminds me of the opening scene to Dances With Wolves. ]
[ MTR: NKF - I snipped some of my long-winded ramblings above, because further testing showed what's up - See my revised "For the record" above. I only ever saw extra points, consistent with what I wrote above. It's not what I had been thinking... you don't get extras if you turn to reaction-shoot, and then don't have enough TUs left to shoot. Also I can only recall seeing it when they still had bullets... I didn't test them as of the turn they ran out of bullets (I had 8 guys shooting 1 alien). Anyway, what I wrote above is very consistent, at least for a firing squad sitation.
It's possible that there are other ways to get extra reaction points. The other concepts such as an alien moving out of or into view (one or the other) can't be as easily tested.
I'll leave this up for a while to see if there's continuing interest and if not, will get rid of most of my comments. And move my stuff on energy-vs.-lighting to Personal Lighting.
Another little note: Even if you only have one person doing reaction shooting, it is possible for them to get the one extra reaction point when an alien goes down. I figure that this happens if your shooter was going to get off two reaction shots before the alien fired. I.e., your guy was queued to fire twice before the alien, but the first one put the alien down.
An X-COM soldier's Reactions are capped at 100. However, because you can get a +6 roll when at 99, a soldier can have up to 105 Reactions (if they're really lucky!). For more info, see Regarding Caps.