Reaction fire triggers

Certain actions will trigger reaction fire from aliens; others will not, even though they may reduce your Time Units. (However, if you have used up several TUs doing "safe" actions then conclude with a reaction-triggering one, an alien with sufficient TUs will make several "stored up" reaction shots against you.)

The following is a (possibly incomplete) list of which actions will and will not trigger reaction fire. Opposing units also need to be facing your unit(s) and have sufficient TUs available; see Reaction fire formula for more details.

Will trigger reaction shots from an alien facing you:

• Firing any weapon
• Kneeling or standing
• Any walking/flying movement
• Using a lift
• Throwing any object

Will trigger reaction shots from an alien not facing you:

• Hitting an alien (they will spin around and return fire)
• Hitting an alien with indirect fire from HE or stun rounds (or even triggering a Proximity Grenade within range)
• Using a Stun Rod or other melee weapon (TFTD) on an alien -- this one is a real oddball: the alien will be "alerted" to your attack, but will not spin and fire until you take one of the actions above that normally triggers reaction shots. Thus you can try stunning them several times, but if you then walk away, they will spin around and shoot. If a different soldier hits the alien with a Stun Rod, the alien's focus will shift to that soldier -- and the first soldier can then safely walk away again. Heh. (Hitting an alien that is directly facing you with a Stun Rod will not trigger reaction shots either, until you perform some other action.)

• Hitting an alien from out of sight distance is similiar to the stun rod. The alien will be "alerted", but will not spin and fire UNLESS you walk within 20 tiles of him, at which point he will suddenly spin around and shoot. This is a pretty rare event though, since it will usually only occur if you had someone firing at an alien from 21 squares away, fire on him, hit, and he survives, then you walk within 20 squares distance (possibly to make way for someone else to fire from 21 tiles away).

Will never trigger reaction shots:

• Turning in place
• Firing a weapon from beyond the alien's visual range
• Firing at a Celatid from beyond its range: 8+ squares indoors (roof immediately overhead) or 17+ squares outdoors is safe (this is due to Celatids' arcing fire trajectory)
• Taking any action (including firing a weapon) from a position not visible to the alien. The visibility is calculated after the the action is taken. For example, if you are visible to the alien, and fire a large rocket between you and the alien, generating a smoke cloud that makes you invisible to the alien, there will be no reaction.
• Moving out of an alien's line of sight -- if after moving one square (or kneeling) you are not visible to the alien, it will not fire at you. This includes moving back one step if you are at the edge of the alien's visual range. If you spot an alien while wandering through smoke, you can move back a step then safely riddle it with holes!
• Priming a grenade
• Firing at (but missing) an alien not facing you
• Hitting an alien not facing you with incendiary rounds, directly or indirectly (combined with the IC explosions causing damage to all units in fire quirk, this is insanely powerful)
• Any action on the inventory screen (moving, dropping or picking up objects; loading or unloading a weapon)
• Using a Medi-Kit, Mind Probe or Motion Scanner

• Mutual surprise:
  • If a unit first sees an alien at the same instant that they first move into an alien's line of sight, the alien won't fire, even if you have no TUs left. The alien you spot doesn't even have to be the alien that spots you -- you can walk through a door and see an alien ahead of you, which will prevent unseen aliens to the side from attacking you.
  • There are cases where aliens can see you (and even fire upon you!), but you can't see them, even if you're facing them (such as walking toward an open door, or when nearing a corner).
  • The "mutual surprise" rule seems to only apply to when you walk (or fly) into an alien's line of sight: if you spot an alien by destroying terrain, or by killing an alien (uncovering another behind it), the newly-revealed alien will shoot at you if it is facing you. An alien will also shoot at you if you spot it when standing from a formerly-concealed position.
  • The mutual surprise rule also does not apply if you move into an alien's field of vision via vertical movement, by using a lift or a Flying Suit. Even if you are facing the alien, you will get shot at as you move up or down into its line of sight.
  • To try to take advantage of this and spot aliens as you round a corner, you should (if possible) approach it at least one space away from the corner and come around at a diagonal. However, there can be a number of "blind spots" near corners, where aliens can see you but you can't see them (see Line of sight#Blind spots around corners).
  • Aliens also benefit from mutual surprise. However, an X-COM unit being tracked by aliens' "intelligence" is already visible, thus cannot trigger the not-visible to visible transition that allows mutual surprise.