How does reflex work?

[Jet]

is it a random check each time? Because i see a reaper come out of a UFO and my soldier shoots him during his turn because i saved enough action points, but the guy next to him does not shoot him. Instead, the reaper proceeds to mutate my commander and THEN the guy who was next to him decides to shoot. Ive reloaded to test and in subsequent plays he shoots a sebilian that came right after the reaper. I had others nearby but they dont decide to shoot or if they DO, then its usually after a reaper sprints across the map.

Is there any way i can influence them all shooting reflexively BEFORE the enemy attacks? or is the reflex shot limited to only 1 character, then enemy walks, then reflex again etc, rotating turns?

[Lobuno]

Is a random roll based on your soldiers reflex stat for each and everyone who is in the correct position against an alien. The only way to influence it is by equipping your soldiers with weapons more prone to reaction fire (i.e. pistols and shotguns). The higher your soldiers reflex stat the better chance you trigger reaction fire, of course.