Metal Gear Solid: Peace Walker/Gameplay

Life and Psyche
The gauge on the lower lefthand corner of the screen is your life. It is depleted by enemy attacks. Once it reaches zero, the mission is a failure. It can be recovered by eating Rations. Keeping a Ration equipped when Life reaches zero will automatically consume a Ration to replenish life. Life regenerates automatically, but it regenerates faster when crouched and even faster when lying down or being pressed against a wall.

Just to the right of the Life gauge is the Psyche gauge. Psyche is damaged by nonlethal attacks. Special attacks by AI weapons will also damage Psyche. Diminished Psyche will decrease accuracy. Psyche regenerates automatically, but it regenerates faster when crouched and even faster when lying down or being pressed against a wall. If Psyche reaches zero, the player will fall unconcious. Wagging the Analog Stick until the Psyche gauge is full is how you escape being knocked out or asleep.

Snake's Life and Psyche are permanent, but MSF soldiers' Life and Psyche may increase for their participation in the Combat Unit. Outer Ops in particular are a great way to increase the Life and Psyche of any MSF soldier. Higher Life and Psyche means more GMP.

Equipped Weapon and Item
On the lower right hand corner of the screen are your equipped item and weapon. Only one of each are used at a time so long as they're equipped. To change equipped item or weapon, open the corresponding menu. Depending on game settings, you may quickly swap between equipped items and weapons with the corresponding item or weapon button.

The weapon displays ammo; white ticks are available rounds while red ticks are expended rounds. The more red ticks you have, the closer you are to needing a reload. The number represents how much ammo you have both in the loaded magazine and in reserve. You can reload by pressing the Action Button on the PSP or the dedicated Reload Button on the HD editions.

Camo Index
On the upper right hand of the screen is the Camo Index, represented by a percentage between -100% to 100% at the extremes. Many variables go into Camo Index and it can change with almost any action taken by the player. Motion reduces your camouflage while staying still increaes it. You'll have a higher Camo Index if you are crouched verses if you are standing straight up. The report of your weapons will briefly but significantly reduce Camo Index.

Camo Index will determine how an enemy soldier reacts if his line of sight intersects with you. In general, the number seems to affect how close you can be before they become suspicious of the player.

Enemy Behavior and Infiltration Status
MSF's operation in Costa Rica is a Sneaking Mission. The goal of gameplay is not to engage directly with the enemy, but to avoid being detected by them. Enemy behavior is state-dependent and the state of the enemy is directly affected by whether or not the enemy has seen you. Normally, the enemy engages in simple patrol patterns or stands guard over an assigned point. Enemy behavior changes when they spot you. Below are the states that govern enemy behavior and how they relate to you the player.

Normal Phase
This is the default state of infiltration. Enemy soldiers are not aware of your presence and are following discrete patrol and guard patterns. They can become "suspicious" if they catch a glimpse of you or if they hear a noise produced by you. If something makes an enemy soldier suspicious, he will investigate the source. If he doesn't see the player during his suspicious phase, he will resume his patrol route. If the player enters an enemy soldier's field of view, the enemy soldier will be startled for about a second before the Alert Phase begins. During this "grace period," the player may attempt to neutralize that enemy soldier to stop the Alert Phase.
 * A suspicious enemy soldier will have a white exclamation mark ( ! ) over his head. The enemy has not spotted you and the Alert Phase will not start.
 * A suspicious enemy soldier who has completed his search without seeing the player will have a question mark over his head ( ? ). The enemy has not spotted you and the Alert Phase will not start.
 * An enemy soldier who has spotted the player will have a red exclamation mark over his head ( ! ) accompanied by a familiar musical sting. If this happens, the enemy has spotted you and the Alert Phase will begin.

Alert Phase
The enemy has seen the player and has alerted his comrades to your location. In the alert phase, enemies will take cover and attempt to assault your position. Alert is also represented by a countdown displayed on the upper lefthand corner of the screen. It will remain at 100 as long as even one enemy soldier has line-of-sight on you. Only after losing the line-of-sight from all enemy soldiers on the map does the Alert Phase count down. When it reaches zero, the enemy moves into the Evasion Phase.

Evasion Phase
During the Evasion Phase, the enemy has a "last known location" for the player and are focusing their search there. Like with the Alert Phase, there is a countdown for Evasion. If the enemy re-establishes line-of-sight with you during this phase, the Alert Phase resumes with a count of 100. Once Evasion counts down to zero, the enemy enters the Caution Phase.

Caution Phase
During the Caution Phase, the enemy gives up on searching the "last known location" and they proceed to methodically search every corner of the map for the player until the Caution countdown reaches zero. When that happens, you return to the Stealth Phase and all enemies will return to their initial patrol routes while any backup called in will leave the map.