During some missions you won't be able to kill anyone, and it may be hard to get close to people to knock them out. To knock them unconscious from afar, equip the SC-20K rifle and the Sticky Cam subweapon. If you hit an enemy in the head with the Sticky Cam they'll automatically drop unconscious. You can pick up used Sticky Cams and use them over and over again.
In multiplayer as a Spy, press and hold the X button to pull out your gun with the laser turned off. Mercenaries can see the laser if you pull the gun out with it turned on. To toggle the laser while the gun is already out, press UP on the D-pad.
When aiming with the pistol, the standard crosshairs won't always cut it. Even though you can't see tell, your aim is always wavering slightly. For more precise aiming, press either UP on the D-pad or pull the L trigger to change your crosshairs to a laser sight.
Note: Enemies will be able to see the laser if it's right in front of them.
When using the SC-20K rifle, click the Right Analog Stick to zoom in. When in this view you'll notice that your aim wavers a bit. To get a steady shot, pull the L trigger to hold Sam's breath and get a perfectly steady shot for a couple of seconds.
If you leave bodies unhidden guards will eventually find them and sound an alarm, even if there are no guards patrolling the area where the bodies are. To hide the bodies, pick them up and move them to a shadow area. Your light meter will flash when it is safe to drop the body. During multiplayer as a Spy, walk up behind a Mercenary and press A to grab him. While holding the Merc, press the WHITE button to whisper threats to him over the headset.
When playing multi-player as a Mercenary, double-tap LEFT or RIGHT on the Left Analog Stick to perform a quick 180-turn.
In single player mode, instead of trying to approach enemies from behind and grab them to knock them out, get close to them--from any angle--and press the R trigger to perform an elbow move and knock them out. Note:Some guards may take two elbows to fully knock out.
During multiplayer as a spy, click in the Right Analog Stick and hold it for a second to bring up the binoculars. Then, highlight a target node and press A to lock on to it. That node will then be the only one that shows up on your screen, letting you focus on it more easily.
When you don't have your rifle yet and you can only use your gun like in the first mission click the right thumbstick to use your binoculars, use the crosshair to aim at the item you plan to shoot, click again to go back to normal view do it gently so not to change your position now whip out your gun and shoot, if done correctly you can hit lights from across the room :D
Second one is very handy, when hiding in a room for an alerted guard use your optical camera to view the other room, now when the guard comes to check out the room, he won't be able to open the door because you're using the camera, instead of freaking out and sounding the alarm he'll try a few times and walk away with a "it must be nothing".