Reaction fire triggers

Certain actions will trigger reaction fire from aliens; others will not, even though they may reduce your TUs. (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 on an alien -- this one is a real oddball: the alien will be "alerted" to your attack, but not actually 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.)

Will never trigger reaction shots:

• Turning in place
• Firing a weapon from beyond the alien's visual range
• Taking any action (including firing a weapon) from a position hidden to the alien
• 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
• 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; also possibly when nearing a corner).
  • If you kill an alien, revealing another alien behind it, "mutual surprise" will not take effect -- the newly-revealed alien will shoot at you if it is facing you.
  • It may be that these rules apply to aliens during their movement turn as well, which would explain why it is better to wait beside a door than in front of it. Or to hide just around a corner.

Currently unknown/untested: