While not wearing a disguise, you can hide behind a desk or other piece of furniture in the room. Remember that an enemy must come around in order to see you. Otherwise, he/she could walk right up to a couch you are hiding behind and he/she will not see you. Note: Going to hide behind a potted plant is the same as standing in the open. You can't hide behind potted plants.
When shot two times (four times with kevlar vest), or pass out from gas, you will be captured. You will then be briefly taunted, and then asked if you would be willing to trade your freedom for one or more enemy agents. Select yes to agree. If you choose no, although you will not have to re-arrest someone, picking the handcuffs takes time. Note: Once you move your character, an alarm will sound and guards will start to arrive within 10 seconds.
There are three types of grenades in the game: Stun (flash-bang), fragmentation (frag), and gas. Never use the short throw command for a frag or flash-bang grenade, as you could suffer damage by being too close to the explosion. Also, frag and flash-bangs will set off an alarm when used in a building. Since gas grenades don't explode and are silent when they release their contents, you can use them without fear of setting off an alarm -- just make sure that you have a gas mask equipped.
To throw enemies off of your trail after being discovered or after lockpicking the handcuffs, close all doors behind you. Since the enemy agent's AI is programmed to look for open doors first, you will gain a small amount of time.
When breaking into a building to make an arrest, search the entire building to find a possible second exit. Before making the arrest, litter your escape route with frag and flash-bang grenades to be set off by remote control. Please note that the remote control will set off grenades you have set for remote control -- make sure that you are nowhere near a grenade when you use it.
To acquire an enemy disguise, just shoot a guard before he can set off an alarm. This allows you to walk among the guards without them knowing who you are. Make sure that you do not look them in the eye, or they will recognize you and set off the alarm. Note: You can not get a disguise after the alarm is set off. You must leave and re-enter the building or remain undiscovered until the alarm turns off. You can tell if the alarm is active by looking at the border of the screen. If it is a solid blue, the alarm is off and you can get a disguise. If it is flashing red, there is an alert and you can not get a disguise.