How does overwatch work in this game?

[Amogus]

Like there is no button to set your soldiers on overwatch like in Xcom, sometime they react fire sometime they don't, Cuz this is really frustrating, when you have a bunch of soldiers get murdered because they don't fire back when the alien shoot them unlike alien who ALWAYS on overwatch for some odd reason.

[The Pollie]

If your operatives have enough Time Units when you end your turn, they can use those on reaction fire when the aliens activate them. It can be useful to tactically station troops at suspected flanks or problem areas, reserve some or even all their time units to fire on approaching threats.

Conversely, having all your time units spent per turn leaves you defenseless if the aliens roll by. Reaction fire is automatic if they have spare time units, nothing on your part but to manage it and ensure they have spare points.

As for Aliens, you can negate their reaction fire by suppressing or flashbanging them. Or just throwing rookies at them to eat the initial fire.

[sePL80]

Adding to the Time Units,
1. every soldier has a Reflex attribute. The higher the attribute, the more likely the soldier is to react to enemy actions.
2. every weapon has a Reaction Modifier.
Pistols and Shotguns f.e. have a modifier of 1.5x
Machineguns and Sniper-Rifles have a reaction modifier of 0.5x.
Rifles have 1x
and Rocketlaunchers do not allow for reaction fire.

I suppose that the modifiers are for your soldiers Reflex attribute.
So when a soldier has f.e. a pistol in hand, half of his Reflex attribute is added. (1.5)
But when he has a Sniper-Rifle or Machinegun in hand, his Reflex attribute is halved. (0.5)

Be careful out there.

[Peter34]

Also, the more Time Units the soldier has left, the more likely he is to Reaction Fire. That is, if a Shotgun shot costs 22 TU, then he is more likely to React if he has 23 TU left than if he only has 22. Slightly. And again slightly more likely to React if he has 24 left rather than 23.

There's also range. It seems to me that even when my Assault Rifle guys end turn with their full TU left, i.e. none used, if React targets are some distance away, my soilders will alwas fire single shots at them, never 3-shot bursts.

That's not really a problem with Assault Rifles, but I imagine it might surprise some players if Shotguns fail to React very often, because they only React, or at are least more likely to React, if the target is within the range.

Assault Rifles are range 20 tiles, but Shotguns are only 8.

[The Pollie]

The system feels hilariously inconsistent, though. I just had an eight-man team go nine rounds straight without triggering a single reaction shot, despite constant alien provocation, and then immediately trigger three of their shots after I make a single tile of movement.

My advice; Don't even attempt to rely on the system. If you are already set up for overwatch, baller. But every time I attempt to use this, I watch aliens run all over me while my troops seem content to ignore anything and everything happening.

[Peter34]

What weapon types are you using?

[The Pollie]

Every type.

[Peter34]

Do you understand the Reflex multipliers?

[The Pollie]

Specifically; Light-scout UFO, shot down and a team sent. Map has been swept and the eight-man team is gathered in positions overlooking the UFO door. I have in total: three riflemen, one shotgunner, one shield/pistol, one machinegunner, two snipers.

Given the sheer amount of firepower pointed at the door and no other threats on the map, I felt comfortable passing a few turns to see if an opportunity doesn't present itself to soften the aliens before I breach. Every turn, like clockwork, a Caesan opens and then closes the door. Nine turns of this, nothing is happening, I decide that I'll just have to go in after them.

Shield swaps his pistol for a flashbang. Plan is to step up to the door, open and flash the crew, then have the shotgunner storm them. I open the door and immediately provoke reaction fire from everyone inside and the breaching team is obliterated before I can do anything.

So, apparently my eight men never triggered it despite each being given nine opportunities, but the two Caesans only needed it once to blow me apart. This isn't the first time I've had an enemy just walk through overwatch unmolested, up until he's already killed two people, then suddenly my men start firing.

[Amogus]

Originally posted by The Pollie:

Specifically; Light-scout UFO, shot down and a team sent. Map has been swept and the eight-man team is gathered in positions overlooking the UFO door. I have in total: three riflemen, one shotgunner, one shield/pistol, one machinegunner, two snipers.

Given the sheer amount of firepower pointed at the door and no other threats on the map, I felt comfortable passing a few turns to see if an opportunity doesn't present itself to soften the aliens before I breach. Every turn, like clockwork, a Caesan opens and then closes the door. Nine turns of this, nothing is happening, I decide that I'll just have to go in after them.

Shield swaps his pistol for a flashbang. Plan is to step up to the door, open and flash the crew, then have the shotgunner storm them. I open the door and immediately provoke reaction fire from everyone inside and the breaching team is obliterated before I can do anything.

So, apparently my eight men never triggered it despite each being given nine opportunities, but the two Caesans only needed it once to blow me apart. This isn't the first time I've had an enemy just walk through overwatch unmolested, up until he's already killed two people, then suddenly my men start firing.

Lol i gave up on overwatch in this game already, for field battle i just slowly craw forward with my entire team untill i reached the UFO or cleared teh map of enemies, then what i do is use guns to shoot open the door or I plant a C4 at the door and boom goes in, cuz those doors are death traps.

[Not Zohan]

Originally posted by The Pollie:

Specifically; Light-scout UFO, shot down and a team sent. Map has been swept and the eight-man team is gathered in positions overlooking the UFO door. I have in total: three riflemen, one shotgunner, one shield/pistol, one machinegunner, two snipers.

Given the sheer amount of firepower pointed at the door and no other threats on the map, I felt comfortable passing a few turns to see if an opportunity doesn't present itself to soften the aliens before I breach. Every turn, like clockwork, a Caesan opens and then closes the door. Nine turns of this, nothing is happening, I decide that I'll just have to go in after them.

Shield swaps his pistol for a flashbang. Plan is to step up to the door, open and flash the crew, then have the shotgunner storm them. I open the door and immediately provoke reaction fire from everyone inside and the breaching team is obliterated before I can do anything.

So, apparently my eight men never triggered it despite each being given nine opportunities, but the two Caesans only needed it once to blow me apart. This isn't the first time I've had an enemy just walk through overwatch unmolested, up until he's already killed two people, then suddenly my men start firing.

As people already said, do not rely on overwatch, it feels like the original X-COM series gives the offensive side the edge. This is no different in Xenonauts.
The game takes the remaining TUs of your own unit into account in calculating the probability of taking reaction fire. That is if you are the target of the aliens.
That means not only is reaction fire (overwatch is just a misnomer in the sense of X-COM) dependant of your weapon, reactions stat of your soldier, but also of the TUs of the alien unit.
More TUs mean that a unit is less likely to trigger reaction fire of the enemy.

For the situation you have depicted, i find it more wise to have a rocketeer, a vehicle or both (version I use) to not let such situations happen.
The vehicle can absorb quite a few hits, and has a lot of ammo, either for suppressing (early) or blowing up cover, doors and walls (after pulse laser).
It also has no downtime between missions, it is always instantly and fully repaired should it survive the mission.

[Quicksilver]

Are your guys actually facing the direction you expect the enemy from? (while soldier is selected, right click the tile you want them to face towards)