Most of the time during combat, you cannot act during another character's turn. If they duck behind a door before you get a chance to shoot at them, too bad: they're behind a door.
There are certain situations, however, where this limitation is actually damaging to the fun of the game. This is most apparent when facing an enemy who has a firearm and is behind cover. One tactic available to them is to raise their head and arm just enough to take a single shot, then duck back into cover. Under the normal combat rules, there is no simple way to counter this tactic: your enemy is behind cover every time you act. Without a grenade or other special ordinance, he is effectively immune to return fire.
The Lay In Wait maneuver allows a character to lay in wait for his enemy to pop out of cover, and then attack him. To use it, the character must sacrifice his Defense and Instant action for that round. If he has applied his defense at any point in the round before to his turn, he cannot use this maneuver. He must then declare where he expects an enemy to appear. If an enemy pops out at the chosen location and takes an action, then the character may attack him as usual (partial cover applies). If an enemy appears at some other location, the character cannot act against them.
The enemy's triggering action can be most any Instant or Movement action. If he's simply peeking out to look, the most you can do is take a reflexive warning shot: you expend one round of ammo and do no damage, but warn the enemy that he's being targeted.