In most encounters, the XCOM soldiers are the aggressors and up against a clock, trying to get their objective done. It is therefore in the best interest of the aliens to engage XCOM in prolonged firefights. To that end, the Better AI includes some routines that check if one of the two major defensive options make sense at the moment: Hunkering Down and entering Overwatch.
The check for these options usually happens after those for the priority shots, immediately before they go into the vanilla target selection.
Overwatch/Hunker Override
The Overwatch/Hunker Override is where the alien checks if it is better off going into Overwatch or Hunker instead of taking crappy shots. It also provides a random chance to go defensive even on decent shots to reduce the predictability of the enemy. Finally, it will make an enemy that is fully alerted, but currently not seeing any targets, strongly consider going into Overwatch and letting the targets come to him instead of going after XCOM themselves – and thus likely running into the player’s own overwatch trap.
The Overwatch/Hunker Override works by checking the highest available hit chance and making a random roll against a percentage. This percentage is further influenced by the number of teammates that are already on Overwatch. If the roll is successful, the alien goes into either Overwatch or Hunker. A second random roll determines which defensive action to take, with higher hit chances making Overwatch more likely than Hunker Down. The exact values are:
- Highest Chance to Hit < 10%
–> Chance to Overwatch/Hunker = 90%/75%/50%
–> 10% Overwatch/90% Hunker - Highest Chance to Hit < 25%
–> Chance to Overwatch/Hunker = 75%/60%/40%
–> 30% Overwatch/70% Hunker - Highest Chance to Hit < 50%
–> Chance to Overwatch/Hunker = 25%/15%/5%
–> 65% Overwatch/35% Hunker - Highest Chance to Hit < 75%
–> Chance to Overwatch/Hunker = 5%/3%/1%
–> 90% Overwatch/10% Hunker - No targets in sight
–> Chance to Overwatch = 75%/75%/75%
(For the chance to Overwatch, the first value is for no other overwatching teammates, the second for one other overwatcher, the third for every one after that)
Some other considerations that apply to use of the Hunker Down ability:
- Enemies get an extra check for Hunker when they are badly wounded or when they are disoriented, increasing their chance to keep their head down.
- Enemies near a VIP or Evac Zone never hunker.
- The Hunker Down ability is exclusively for ADVENT units. The alien races are not able to use it. When an alien would want to hunker down, it will instead go into Overwatch.