Instead of activating you can put a unit on Overwatch which ends their actions this turn. During your opponent’s turn, you may interrupt their activation to perform overwatch actions. You must declare which actions the unit will take when the unit goes on overwatch – the order does not matter (Fire/Move can also Move/Fire) Note if you move first and then go on overwatch, you will only get 1 activation during your opponent’s turn.
Also, you don’t have to tell your opponent what kind of overwatch you’re using. Put your tokens face down and reveal them when you activate. If, at the end of your opponent’s turn you have units on overwatch, you can elect to activate them and carry out the actions you had put them on overwatch for (ie, move or if they have LoS – Fire) If the unit on overwatch is pinned during the turn, they will no longer be considered “on overwatch” and will not be able to interrupt their opponent’s turn.
You cannot activate a unit on overwatch during the same player turn the unit first went on watch (Ie, you cannot go on watch, then interrupt an enemy who is reacting to one of your other units if it’s during the same turn you placed the unit on overwatch. However, if your unit remained on overwatch from a prior turn, you can interrupt an enemy overwatch action during your turn.)
Example – Unit A is firing at Unit B that is on overwatch (move) with one token. Unit A declares a fire-fire action and unit B elects to interrupt and move out of LoS. Unit A no longer has a target and is free to re-assign their actions and continue their turn. If Unit B had a fire token, they would interrupt Unit A and put a round of fire on them. If, as a result of their fire action Unit B gets pinned down, they would need to spend their first action attempting to un-pin before returning fire.
Note – vehicles as well as infantry units can be on overwatch