Metal Gear Solid: Peace Walker/Gameplay

Gameplay
Metal Gear Solid: Peace Walker is a stealth game. The main interaction between the player and the challenges of the game is sneaking around them rather than direct engagement. To that end, the player is equipped with tools to aid in the task of remaining in stealth such as non-lethal tranquilizer guns, camouflage, the ability to move slowly without sound, or to crouch and move at a lower profile.

Rather than the on-site procurement and upgrade of stealth tools that had traditionally characterized the Metal Gear franchise, Metal Gear Solid: Peace Walker introduces Mother Base from which Snake's equipment is developed by the staff under his command.

Missions and Mother Base are two distinct core gameplay modes that compliment one another. As Mother Base grows, so do the capabilities of Snake and his men when executing Missions. As Snake and his men complete missions, so does Mother Base grow.

The core interaction between Missions and Mother Base is the recruitment of soldiers through the use of the Fulton Recovery System, and the core interaction between Mother Base and Missions is the number of weapons and equipment at the player's disposal.

User Interface
The gauge on the lower left-hand 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.
 * Life and Psyche

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 unconscious. 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.

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.
 * Equipped Weapon and Item

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.

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

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.

Player and Enemy States
When an enemy is active and can respond to your presence, is a threat. When you have changed his state to any of the ones below, he has been neutralized. The following are the game's distinct states of neutralization. Stunned enemies were neutralized by CQC, Stun Grenades, and Stun Rod, among others. The number of stars over his head signifies how close he is to getting back up; the smaller number of stars, the sooner he becomes a threat. The maximum number of stars is five, usually inflicted by the Stun Rod's full charge. Sleeping enemies were neutralized by tranquilizer weapons such as the Mk.22 and Mosin-Nagant. The number of Zs over a soldier's head signifies how close he is to getting up; the smaller number of Zs, the sooner he becomes a threat again.
 * Stun (★★★)
 * Asleep (ZZZ)

You can make Stunned or Sleeping enemies come to by holding down the to perform a Body Check. Every time the patting animation plays, a star or Z will be knocked off and the neutralized enemy will be that much closer to becoming a threat again. Enemies are brought to this state by lethal force. After a few seconds, an enemy Near Death will die. Other enemy soldiers can resuscitate enemies in this state. You can also resuscitate them with the Stun Rod - effectively a defibrillation - overriding Near Death with Stun. Killing enemies and/or leaving them Near Death disqualifies you for an S-Rank and subtracts from your Heroism score. Enemy soldiers extracted with the Fulton Recovery System while in this state will go directly to Sickbay. There are certain missions during which the use of nonlethal force will cause them to enter a state similar to Near Death; they will be neutralized with a 💀 over them. This is due to hardware limitations imposed upon that mission. Enemy soldiers will still "die" and disappear from the battlefield as if they were kills, but enemies brought to this state do not count as kills or enemies brought Near Death. When you point your weapon at an unsuspecting enemy soldier, you will neutralize them with a Hold-Up. They will lie down on the ground, having surrendered. They will never leave this state unless another enemy soldier has seen you and forced and Alert Phase or another enemy soldier has seen a soldier in this state.
 * Near Death ( 💀 )
 * Surrender

Only enemy soldiers who have been neutralized into any of the above states can be extracted with the Fulton Recovery System and recruited into MSF.

The above states can also apply to the player under certain circumstances. When the player has lost all of their Psyche, they will become incapacitated - either Asleep or Stunned depending on the attack that finished off the player's Psyche. Once this happens, you will be required to waggle fill your psyche gauge and shake off the stars or Zs until you get back up. Like with enemies, the number of stars or Zs represents how close you are to being able to get back up. If you're playing in CO-OPs or Versus OPs, losing all of your life will bring you to Near Death status during which you can be resuscitated by another 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. 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. 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. 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. 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.
 * Normal 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
 * Evasion Phase
 * Caution Phase