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, IC, 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
• Priming a grenade
• Firing at (but missing) an alien not facing you
• 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
• Being the first unit to enter an alien's line of sight. This means you can safely walk through a door, even with no TUs left! It's your next action that will get you killed. (Subsequent units which enter the door will also get shot at -- the alien is no longer "surprised".)

Currently unknown/untested: