Reaction Fire
Current Reaction Score = (Reactions Stat) × (Current TUs / Max TUs)
Your reaction score at a given moment is your reaction stat multiplied by the percentage of your remaining TUs. As you spend time units on any actions, your reaction score goes down.
A rookie camping in one location and not spending Time Units will usually have higher Reactions than a trained officer who is exhausted of time units after sprinting across the map.
How is it used? There are two inherently identical uses. During your turn, and at the end of your turn. You can think of reactions as having two different roles: defensive and offensive.
During your turn, when your soldiers are given orders, each unit's current reaction score is directly compared with the current reaction score of any alien that can see the unit. If the alien is looking the other way or there are obstructions, it will not be able to react until it is alerted.
Given these conditions, the game constantly checks the unit's reaction score against the alien's. The side that has the highest reaction score can keep making moves without interruption. The moment the opposing unit has the higher score, that unit will be able to interrupt the first unit and is given an 'attack of opportunity' - or reaction shot.
At the end of your turn, your positions are reversed. The defenders are now the attackers, and the attackers are now the defenders. When the aliens get to move, if their reaction scores are below yours and you have sufficient time units to return fire, you get a reaction shot.
Note: during the alien's turn, the aliens are allowed to use auto-shots, as it is their turn. Reaction shots are always performed as snapshots for X-COM owned units. AI owned units on the other hand can use either snap or auto shots when performing an attack of opportunity.
Health and Fatal Wounds will also affect reactions.
[ NKF : I don't seem to recall finding that health or wounds affect reactions. Hmm, this needs to be looked in to further and confirmed. ]
Tip: Use the Mind Probe to determine if an alien is in fact out of Time Units. Take notice of the amount of remaining time units. This can give you a good indication of whether or not the alien will be able to react should you need to shoot, capture or approach the alien to strike it with the stun rod.
A Working Example:
Werner Okamoto has a modest reaction level of 30. He has 60 time units in total. He is armed with a Laser Rifle, which can fire snapshots at 25% of his time units per shot (or 15 TUs a shot). He ends his turn without spending any of his time units.
Generic Nasty Alien Shaped Thing has a massive reaction score of 70, and 50 time units. For fun, it's carrying a Heavy Plasma! It fires snapshots at 20 TUs per shot. It is exhausted and only has 50% of its TUs left (25/50). He is staring right at Werner.
The human turn is ended. The aliens now get to choose what to do. Naturally, since the alien can see the soldier, it will attempt to attack.
Move 1:
Werner = 30 * 60/60 = 30 * 1 = 30
GNAST = 70 * 25/50 = 70 * 0.5 = 35
The alien has the higher score and takes a shot, spending 20TU, leaving 5.
Move 2:
Werner = 30
GNAST = 70 * 5/50 = 70 * 0.1 = 7
Not only does the alien not have any time units left for the shot, it fails its reaction score. Instead, Werner takes a shot, reducing his TUs to 45.
Move 3:
Werner = 30 * 45 / 60 = 30 * 0.75 = 22
GNAST = 7
(Werner shoots again, spending 15 more TU)
Move 4:
Werner = 30 * 30 / 60 = 30 * 0.5 = 15
GNAST = 7
(Werner shoots again, spending another 15 TU)
Move 5:
Werner = 30 * 15 / 60 = 30 * 0.25 = 7
GNAST = 7
Although equal, Werner can still make his 15TU shot, but the alien cannot make a 20TU shot. Werner takes the shot. As this exhausts Werner's TUs to 0 / 60, the exchange of shots ends right here.
Adopted from discussion by NKF, Jan 12 2004 --JellyfishGreen 16:33, 21 Apr 2005 (BST)
Additional updating by NKF, 28 September 2005