Tilt the left stick a little to move slowly and all the way to move quickly.
The faster you move, the easier it will be for enemies to spot you.
Basics of Infiltration
You will be at a distinct disadvantage if the enemy spots you. Move as stealthily as you can to ensure you are not discovered. Enemies also respond to noise.
The lower your stance, the less likely it is that you will be spotted.
Use the left stick to move in any stance.
Approach an object to stick to it and conceal yourself.
You can even move along the object while staying hidden.
While behind cover, hold the Ready Weapon button to pop out and take aim (only if the cover allows). Release the button to return to cover.
Lean over a cliff edge or other ledge to grab and hang off it.
Use the left stick to move while hanging.
What to Do When Spotted
Enemies that spot you will notify their allies by shouting, calling on the radio, or 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 time carefully searching the area where they last saw you.
The moment you are spotted, the action slows down (Reflex Mode), and you can point your weapon at whoever spotted you.
If you neutralize whoever spotted you while Reflex Mode is still in effect, you can prevent them from notifying their allies.
Perform a quick dive in the direction you are tilting the left stick.
This is useful for evading enemy attacks or their line of sight.