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. Additionally, 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's 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 AI Director.

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 1/3 of the time if playing with mediocre teamwork. On normal, pain pills are given out more than health kits and tier 2 weapons are less likely to be found. Infected are stronger, Tanks and Witches are harder to kill and spawn slightly more.

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 lead to high casualty rates. Expect to fail finales approximately 25% of the time and around 50% of Survivors will not survive a finale. Typically, one Survivor out of the four will not survive the finale, and at least one Survivor will be downed every chapter.

Expert: On expert, the Director has no mercy. Health kits cannot be found outside of the safe room (except for finales and the ambulance right outside of Mercy Hospital); pain pills become quite common, tier 2 weapons are difficult to find in-game. Common Infected are deadly, Witches kill you in 1 hit, Tanks can incapacitate you in one hit and kill you in 2-3 hits, and the horde will arrive much faster. 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.

Adaptation
The Director can adapt to any existing map, including those initially developed without Left 4 Dead's event markers, as seen in this video of the Counter-Strike: Source map de_dust in the demo.
 * In Left 4 Dead 2, the Director is capable of changing maps themselves; turning the maps from simple, straight-forward 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. And, if the spit manages to miss, the Director will begin damaging you by 10 damage per second when you are in an area you are not supposed to be.  Examples include behind the Plantation house (using the Grenade Launcher) and atop the roof of the building in the Bus Depot using the pipes to get up there.

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 a infected or stand on the map,you will look like a giant on the actual map. You can also see miniature common, Special Infected on the map as well and being spawned. Because of this, people who find this out call it "The Directors Chair" due to the face seeing this is where the AI controls the map.

Reading the AI Director
With a bit of practice, it is possible to anticipate moves by the AI Director. For example, if you are easily dispatching hordes during panic events, expect the AI 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.

Versus mode
The AID 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. Pipe bombs, molotovs, pills, and first aid kits, are also identical for each team in terms of placement and number, though originally they were not.