This is not my idea originally, but I really like it, although I haven't entirely worked up the details of its mechanics yet.

This feature is going to be activated by a hot-key ("k" probably) similarily to using "l" to turn around and raise your gun. When you hit the key you got a cursor to point at either an enemy or an empty tile. When you do that, your merc gets "locked on" the enemy/tile. This means he got all his attention at the specified spot, which allows him to get almost certain interrupt at critical moments. For example - locking on the tile at a corner of a building means you are all ready and focused to pull the trigger the very moment any enemy pops up there. When you are locked on an enemy you get an interrupt at every critical action he is going to do - when he spots you, when he raises a gun, etc.
This feature would interact with our Interrupt Counter thingy - the interrupt would be automatic if the Counter is already filled up (but put on "wait" because of lock-on feature); or if the counter is not filled up yet, there would be a chance calculated at the key moment based on the current Interrupt counter value and merc abilities.

When "Lock-On-Enemy" I would go to make the merc automatically turn when the enemy watched moves out of LOS and turning would allow the merc continuing to watch him (i.e. the next tile enemy moves is visible from mercs position).

The "Lock-On-Tile" feature not only applies to the single tile, but also tiles around it, however, with lesser chance.