MoveLeft stick
Tilt the left stick a little to move slowly and more to move quickly.
The faster you move, the easier it will be for enemies to spot you.
Sneaking 101
You will be at a distinct disadvantage if the enemy spots you. Sneak as carefully as you can to ensure you are not spotted. Enemies also respond to noise.
Change Stancebutton

The lower your stance, the harder it will be to spot you.
Use the left stick to move in any stance.
Hang

Hang
You can grab on to the edge of a cliff or other ledge when you lean your body toward it.
Use the left stick to move while hanging.
Climb back up with the button. Let go and drop down with the button.

Hang
What to Do When Spotted
Enemies that spot you will notify their allies by shouting, calling on the radio, or some other means.
Take the appropriate action for the situation, such as eliminating the enemy, fleeing, or hiding.
If a battle is underway, reinforcements might still show up even after you eliminate the initial enemy.
If you give enemies the slip, they will spend a while carefully searching the area where they last saw you.
Reflex Mode
The moment you are spotted, the action slows down (Reflex Mode), and you can press the to point your weapon at whoever spotted you.
If you neutralize whoever spotted you while the Reflex Mode is still in effect, you can prevent them from notifying their allies.
Quick DiveLeft stick + button
Evade in the direction specified by the left stick.
This is useful for evading enemy attacks or their line of sight.