The Director

The Director, sometimes referred to as the AI Director, or simply as AID is the artificial intelligence of Left 4 Dead that features a dynamic system for game dramatics, pacing, and difficulty.

Instead of set spawn points for enemies, the Director places enemies in varying positions and numbers based upon each player's current situation, status, skill, and location, creating a new experience for each play-through. The Director also creates mood and tension with emotional cues such as visual effects, dynamic music and character communication. Moreover, the Director is responsible for spawning additional health, ammo, weapons, and Special Infected, like the Witch or the Tank.

It should be noted that there is another Director in the game, which controls the music on a per-player scale, called the Music Director. This article will only deal with the main one, the Director in charge of spawning enemies and items.

The Director's Mercy
In game, the Director has certain ways of showing mercy (or cruelty) to the players, such as:

Easy: On easy, the Director will have pity, giving you health kits and Pain pills more frequently. The Tier 2 weapons will be given earlier in the game. Infected spawn normally and Tanks and Witches are less likely to spawn. Typically, there are very few incapacitations on Easy, and deaths are very rare. Missing the rescue vehicle is rare and all Survivors will probably survive, even if all are new players.

Normal: The Director here gives you an even chance of failure or success. Good playing will be rewarded, and bad playing will be punished. Survival is not guaranteed, as you will probably be incapacitated at least once and usually at least one person will miss the rescue vehicle if playing with mediocre teamwork. On normal, Pain pills are given out more than health kits are and Tier 2 weapons are less likely to be found. Infected are stronger; Tanks and Witches are harder to kill and have a higher chance of spawning more often than on Easy mode.

Advanced: On Advanced, the Director is out to test your resolve. Health kits are rarely found and Pain pills are hard to find as well; Tier 2 weapons are not found until later on in the campaign. Infected are much stronger; Tanks and Witches are tougher to kill and spawn in almost every chapter from the second chapter onwards, usually on the path you take. The lack of health supplies coupled with the 5 points of damage from Common Infected attacks leads to high casualty rates.

Expert: On expert, the Director has no mercy. Health kits are almost impossible to find outside of the safe room (except for finales and areas where they always appear); Pain pills become quite common, Tier 2 weapons are difficult to find in-game. Common Infected are deadly (20 damage per hit in the front and 10 damage if they attack on the back). Special Infected are even more deadly (watch out for Hunters, which can deal up to 40 damage with a single claw attack), Witches kill you in 1 hit, Tanks can incapacitate you in one hit and kill you in 2 to 3 hits, and the horde will arrive much faster, especially if the Director thinks you're moving too slowly. Expect to die in every chapter, get rescued from rescue closets, and dying in finales often meaning that you will not make it to the rescue vehicle.

In Left 4 Dead 2, the Director's humor has become "sicker," because it will spawn Witches and Tanks often, no matter what difficulty you are on. Although their health depends on the difficulty, players may become annoyed when they spot a Witch and Tank at the same time. The Director also seems to have become more "stupid", for it may spawn Special Infected in walls, out of bounds, in rooms which cannot be entered or left, or just out in front of the Survivors. It also seems to spawn Common Infected in the open. Some players may have an Infected spawned right before their eyes.

If the Director is feeling especially cruel, it may spawn a Tank or Witch (or both when the Director is feeling exceptionally mean) within the safe room the Survivors have to go into. This is far more common on Advanced and Expert to exceptionally well-coordinated teams. Usually during these situations (especially during a gauntlet event such as the fourth part of Dark Carnival), the Survivors take a severe beating and are forced to become victims of the Director's dark sense of humor.

Adaptation
The Director can adapt to any existing map, including those initially developed without Left 4 Dead ' s event markers.
 * In Left 4 Dead 2, the Director is capable of changing maps themselves; turning the maps from simple, straightforward walks to seemingly endless mazes that are riddled with dead ends. This change depends on how well the Survivors are doing, like the changes the original AI Director from Left 4 Dead did; for example, in The Cemetery, players must navigate the Survivors through a mausoleum-filled cemetery, which can be altered to the AID's liking. Other, smaller changes (such as certain areas and entrances being blocked by fences) are noticeable, too, but aren't as devastating as the area-specific alterations.


 * In Left 4 Dead 2, when playing Survival Mode, the Director has the ability to punish cheaters by spawning Spitter acid patches on anyone who is outside the map and/or cheating from the time the radio is started to the time you die.


 * In Left 4 Dead 2, the Director is sensitive to the amount of teamwork going on as measured by how many times teammates heal each other and rewarding more teamwork with more health kits throughout the map.

Manifestations
Here are some of the game manifestations that explains the Director being able to change map layouts; These manifestations were proven by multiple playthroughs

Left 4 Dead 2

 * The Hotel--the first level of the Dead Center campaign--The Director has the ability to provide hints for direction, especially if it is the first time (or first hours) of the player in the game. The Director will cast flames on the ground to block the player if they are going on the same place for a long time. However, if the game has been played more often, these flames will not be spawned in the next playthroughs.
 * On the 3rd level of the Dead Center campaign, The Mall, the Director can block the doors with vending machines and can open up a door leading to another direction. In fact, the event that can cause the alarm on the 3rd floor to activate may vary; One direction allows you to open a door, and another route requires you to break a store glass.
 * On the Dark Carnival's chapter, The Coaster, The Director might hide or alter the locations of the maintenance ladders needed to get back up to the track, in the event that a party member falls off.
 * The Director can alter the maze of the Cementery level of The Parish campaign, in which the Survivors may find themselves lost upon the second playthrough.
 * In the first part of the Dark Carnival campaign, The Highway, players may be able to find the steep slope and a river just beside the Motel, in which Coach or Nick may refer to it as "a shortcut".
 * Another manifestation in the Highway chapter of the Dark Carnival campaign is that the Director is able to block the roads with multiple cars and buses.
 * In the Swamp Fever campaign, the Director can spawn bushes and trees to block the players' paths.
 * Depending upon the Director's 'mood', the storm in the Hard Rain campaign may increase and the screen can even get darker. It has been proven true that the better your team is doing, the higher of a chance the Director will create more rain.

Controlling The Director
Using the console command, it is possible to take on the role of the Director, spawning Infected, forcing Panic Events, and various other features. When running a dedicated server, the administrator of that server can create a controlled environment by turning the AI Director off and controlling all of the aspects using console commands. If you use Noclip and fly down to the skybox, if you spawn an Infected or stand on the map, you will look like a giant on the actual map. You can also see miniature Common and Special Infected on the map as well and being spawned. Because of this, people who find this out call it "The Director's Chair" due to seeing it is where the AI controls the map.

Reading the AI Director
With a bit of practice, it is possible to anticipate moves by the Director;


 * For example, if you are easily dispatching hordes during panic events, expect the Director to send in some Special Infected to increase the difficulty.


 * If you pass multiple areas with very few Infected, it is likely you will be dealing with a Witch or a Tank soon. Watch out for the Director's nasty sense of humor; once in a while, it will spawn a Witch right in front of a vital passage.


 * Bear in mind that the Director will sometimes increase the amount of Infected above the normal amount for the difficulty level if the player is very experienced.

Time spent in the game
The Director can even record the total amount of time being spent in the game. Likewise, the Director would "guess" the experience of the player due to the time spent in the game. If the game was continously played within hours, then the Director expects that player to be experienced enough.

One can see the examples while playing the game on the same difficulty several tries. Here are some gameplay variations and changes as the game is played on longer hours, depicting the Director to challenge the players:
 * Smarter Special Infected; Jockeys lead you to Witches, Spitters spitting on incapacitated Survivors, and Boomers puking those who were captured by Hunters and Chargers.
 * More Special Infected in Crescendo Events
 * Extended number of Special Infected on the same difficulty. Even a Witch can spawn near the start of a level.
 * If the Director finds the game too easy for experienced players, Chargers, Smokers and Tanks appear several times.
 * The Director changes the levels' outlines at will (see manifestations above). Also, new music during Crescendo Events will be played

Versus mode
The Director is also easy to read in Versus mode because the Director tries to be somewhat fair to each team. Tanks and Witches for both teams will always spawn near the same points in the map, though the Tank for the second team playing can/will arrive slightly earlier or later. In some levels, this can have devastating results for either team. For example, in The Hotel: The Tank for one team spawns before the elevator and can kill the Survivors; then the Tank for the second team spawns after elevator and will die due to frustration. Pipe bombs, Molotovs, Pills, medkits, and weapons are also identical for each team in terms of placement and number, though originally they were not.

Rewards and Punishments
The Director can even reward the team based on their actions on the game.

Rewards
The Director will reward the players if they are playing clean, i.e. with such actions as no friendly fire, protecting teammates, healing, reviving, and helping other teammates will reward good items like the following:
 * Grenade Launchers will spawn on the middle areas of the game, players may also be surprised to find a Grenade Launcher in Safe Rooms
 * Healing items (Health Pack, Adrenaline Shot, Pain Pills) will appear more often to reward the players of a good gameplay
 * More Tier 2 weapons will appear.

Punishments
The Director can also punish those who play dirty, i.e. such actions as friendly fire and leaving the team can be punished with the following:
 * For players who separate from the team and moves on to another route will be punished; The Director will spawn Special Infected to the players who disband from the team.
 * For players who hit their teammates, The Director will punish them by spawning Hordes, to attack that player.