Reaction fire triggers

Reaction fire is triggered through a variety of actions. There are also some actions that do not trigger, or immediately trigger reaction fire. The following is a (possibly incomplete, with some duplication) list of what actions will and will not trigger reaction fire.

Note, all of these assume that the requirements for a Reaction shot are all met by the defending side. Also while the lists are focused on the X-COM side, the conditions apply to the aliens as well.

Triggered while alien facing towards you

Action	Notes
Firing any weapon	Hit or miss, the alien will react if it survives. Note that line of sight calculations are done after the attack. If you fire an explosive round and create a smoke cloud dense enough to obscure you from the alien's vision, it will not be able to react.
Any walking/flying movement
Kneeling or standing
Using a lift
Throwing any object

Triggered while alien facing away from you

Action	Notes
Hitting an alien	The alien 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 melee weapon	This does not immediately trigger a reaction shot but the alien will be "alerted" to its attacker. The alien will only react if the soldier then performs any other action that will trigger a reaction shot. This means a soldier can safely attack an alien several times with a Stun Rod, but if the soldier then walks away the alien will immediately turn around and shoot. If a different soldier hits the alien with a Stun Rod, the alien's focus will shift to that soldier. The first soldier can then safely walk away.
Hitting an alien from beyond visual range	Similar to the melee attack, the alien will be "alerted", but will not spin and fire. If the last person to shoot and hit the alien walks into its visual range on the same turn, the alien will immediately turn and fire.

Never triggers reaction shot

Action	Notes
Turning in place
Firing beyond the alien's visual range	If the shot hits, the alien will be 'alerted' and will react if the soldier comes within its visual range. If the shot misses, it will not be alerted.
Firing at a Celatid / Deep One from beyond its range	8+ squares indoors (roof immediately overhead) or 17+ squares outdoors is safe (this is due to the arcing fire trajectory)
Moving out of an alien's line of sight	Any movement or kneeling action that immediately obscures a soldier from the alien's sight. Such as stepping behind a wall or taking a step backwards into a smoke cloud.
Priming a grenade
Hitting an alien not facing you with incendiary rounds.	Directly or indirectly. Due to the quirky nature of incendiary attacks, combined with the IC explosions causing damage to all units in fire quirk can be very powerful.
Any action on the inventory screen	Moving, dropping or picking up objects; loading or unloading a weapon
Using tools	Includes: (UFO) Medi-Kit, Mind Probe, Motion Scanner & Psi-Amp. (TFTD) Medi-Kit, M.C. Reader, Particle Disturbance Sensor, M.C. Disrupter.

Note: If you have used up several Time Units performing the above "safe" actions and then conclude with a reaction-triggering action, an alien with sufficient TUs will make several "stored up" reaction shots against you.

Mutual Surprise

• 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.