During a Break-In, it is possible for Max to acquire a disguise from an enemy guard. This involves killing or disabling the guard, then taking the guard's uniform.
A Disguise can allow a careful player to navigate through the Hideout with much less danger of the alarm being raised.
However, the ruse can fail if the player is not careful. If it does, the alarm will be raised, and the disguise is lost. It is then possible to get another disguise in the same way, once the alarm has died down.
Acquiring a Disguise[]
First, you must kill or disable a guard before you can obtain their disguise. (Your gun and frag grenades kill, while stun grenades and a few seconds' exposure to gas will disable a guard - temporarily.) Once the guard changes into the picture of a sprawled-out, motionless body, move Max directly on top of the guard to take the uniform.
This will only work if the alarm is not activated at the time. This is easily discernable, as the alarm makes the screen flash red. If it is, do not walk over the body if you wish to take the disguise, otherwise it will disappear and leave nothing behind.
Once a disguise has been obtained, the picture of Max in the top-left corner of the screen will show him wearing a colored shirt of the same color that the enemy guards wear (instead of his normal black shirt). In addition, the word "Disguise" will be flashed in green over the game clock, at the top-center of the screen.
Using a Disguise[]
A disguise is used for walking past enemy guards without them raising the alarm or firing at Max. This means Max no longer needs to hide behind furniture and crawl his way whenever a guard enters a room or whenever Max needs to go past a guard to get somewhere.
In fact, while wearing a disguise it is even possible to walk straight into the Agent Room (if there is one), without causing the sirens to go off. That's something which is all but impossible when not wearing a disguise at all.
However, a disguise is definitely not a free pass - it does not simply allow traipsing right in front of the guards. There are certain conditions which must be observed to avoid blowing a disguise and raising the alarm.
Facing[]
While Max can wear an enemy's uniform to become less noticeable, his face remains unconcealed. This means that if an enemy guard gets a good look at Max's face, he will recognize Max and raise the alarm immediately (proceeding to attack max right away).
Whenever Max and an enemy guard are in the same room and facing each other (or up to about 90 degrees away), the guard will stop in his place and appear to draw a gun on Max. While this may look very hostile to a novice player, this is just a warning period.
At this point your best move is to turn away immediately, facing away from the guard. If you did this quickly enough, the guard will lower his weapon and walk away. As long they can't see Max's face, guards will disregard max entirely, as though he wasn't there.
If you fail to turn away in time, the guard will raise the alarm and proceed to attack.
Note that if the guard has his back to Max, facing is not relevant. Max can walk up behind a guard who's, for instance, inspecting a door or a piece of furniture. If that guard turns around, of course, Max has to quickly turn away himself, as explained above, to avoid being recognized.
Suspicious Activity[]
One easy way to lose a disguise is to perform any suspicious activity while a guard is watching. If the guard has a clear view of the activity, he will immediately raise the alarm.
Suspicious Actions include, most commonly, the Photography of Containers, as well as the Bugging of Buggable Furniture. If a Guard is looking at Max's direction at the time (or at most about 90 degrees off Max to either side), he will raise the alarm immediately without a warning period.
Note that you can still perform Photography and Bugging behind a guard's back. As long as they can't see you perform the action, it will not raise the alarm or blow your disguise.
Note that opening and closing a container is not considered suspicious. Enemy guards do it all the time as part of the routine patrol, so if they're assuming you are one of them (thanks to the disguise), they would find nothing suspicious about that sort of thing. Note that if you leave the room while a Container is still open, any guard spotting the open container will still raise the alarm, so make sure to close everything behind you as you go.
Hacking a Terminal follows the same rules as above. Also, failing to enter the correct password will also raise the alarm, whether or not you're in disguise. Note that even successful completion of the Hacking process may cause the alarm to go off. This depends on your chosen Difficulty setting.
In contrast, it is quite possible to extract a Password letter from a Terminal without raising suspicion, even inside the Agent Room (see below).
Killing Guards[]
In Covert Action, gunfire is apparently 100% silent. If the alarm is not activated at the time, guards will completely ignore any gunfire, even if it is directed at them.
This means that while in disguise, it's quite possible to ambush enemy guards and kill them. There are plenty of ways to do this, probably the best being to face away from the guard until he comes close, then turn around and shoot him full of lead. You could even let him pass and then shoot him in the back as he walks away from you. There are various tricks, too many to list here anyway.
However, a dead body is, not surprisingly, very suspicious. In fact, if any guard spots a dead body, he will immediately raise the alarm. This means that if you're trying to keep your disguise, you need to hide the bodies. This is done by walking over the body of the dead guard, just as you did to acquire the disguise in the first place.
This rule about dead bodies is the main reason why it's usually a bad idea to kill a guard when you can't easily and quickly reach his body. It also means that attacking a guard while another guard is nearby is a big risk - if that guard turns around or gets a clear view of the body, the alarm goes off instantaneously.
Grenades[]
One sure-fire way to raise the alarm is to throw a Frag or Stun Grenade. The explosion will rock the building, and the alarm will be raised immediately. It doesn't matter whether anyone was hit by the grenade, or whether all the guards are on the other side of the building at the time - the alarm will be raised, and your disguise immediately lost.
Gas Grenades curiously don't cause this, even if guards walk straight through the room affected by the Gas.
Planting Booby Traps and Remote Bombs does not by itself set off any alarm, regardless of the type of grenade used in the process. Both will remain hidden until triggered (the former by enemies passing next to it, the latter by Max's design). Once the trap/bomb is triggered though, if it is made of a Fragmentation or Stun Grenade, the explosion will raise the alarm. Again, a Gas Grenade will not raise the alarm at all.
Enemies throwing grenades can also trigger the alarm in the same way described above - when throwing Frag and Stun Grenades. Usually however, they won't do this unless the alarm is already sounding. The real effect is that they simply reset the alarm timer, causing the existing alarm to continue for a longer while. In other words, there was no disguise to lose in the first place, since the alarm was already blaring.
In the Agent Room[]
Raising the Alarm inside the Agent Room (if there is one) is very common. There's very little that can be done in that room at all without raising the agent's suspicion. In fact, just walking into that room without a disguise will automatically raise the alarm.
In a disguise, however, things are a little different.
Facing[]
As long as there are no guards in the Agent Room, Max can freely walk around the room in his disguise. He can even stand face-to-face with the agent himself for as long as he wants, without raising the alarm at all. Again, if an enemy guard ever comes into the room, you must obey the normal rules for guards as outlined above.
Suspicious Activity[]
If Max ever performs a Suspicious Activity in the Agent Room, the agent himself will raise the alarm immediately.
This is similar to a "normal" guard raising the alarm when he spots Max engaging in suspicious activity, and the same set of Suspicious Activities apply here as well. Just note that an Agent has a 360-degree vision, so performing actions "behind his back" doesn't help at all.
Arresting the Agent[]
To perform an Arrest of an agent, Max must walk up to that agent and press the "ENTER" key, then reviews the Suspect File for this agent and decide whether to arrest him or not.
Regardless of whether Max confirmed the arrest, the alarm will go off. In other words, simply walking up to the agent and perssing the "ENTER" key will immediately raise the alarm.
If the agent has been nabbed out of his chair, it is now entirely impossible to acquire another disguise, even after the alarm dies down. This is one of the reasons why an Arrest should, if possible, be performed at the end of a Break-In, because things get more difficult once the deed is done.
Confiscating an Item[]
Strangely enough, Confiscation of an Item from the Agent Room, though done right in front of the agent himself, does not raise the alarm at all.
This means that confiscation is best performed when in disguise. You can walk freely into the Agent Room, grab the Item, and leave, without raising anyone's suspicion. This is pretty much a "clean getaway".
Of course, if you also Arrest the agent himself, that will raise the alarm (see above).
Acquiring Another Disguise[]
If your first disguise is lost due to the alarm going off, you can still acquire another one, and another, as many times as the alarm is raised.
To do this, wait until the alarm dies down, find another guard, kill him, and take his uniform. This is the same process as when you got that first disguise.
Note that it doesn't matter when the guard is killed - only when his body is picked up. In other words, you can kill a guard while the alarm is sounding, wait for the alarm to die down, and then pick up the body to get another disguise. Of course, remember that if any other guard walks in and sees the dead body, they will reset the alarm timer, making the alarm last longer.
In contrast, picking up any dead body while the alarm is still blaring will simply hide the body without acquiring a disguise.
The only exception to this is after the agent residing in the current Hideout (if there is one at all) is Arrested. Once the agent is nabbed, it becomes impossible to gain another disguise until you actually leave the building. After all, you are carrying the nabbed agent!