The Director

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

Easy: On easy, The Director will have pity, giving you health kits and pain pills more frequently. Tier 2 weapons will be given earlier on in the game. Infected spawn normally and tanks and witches are less likely to spawn.

Normal: 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, health kits are rarely found and pain pills are hard to find as well, tier 2 weapons are not found till later on in the Campaign. Infected are much stronger, Tanks and witches are tougher to kill and spawn more often, usually on the path you take.

Expert: On Expert, The Director has no mercy. Health kits are extremely rare outside the safe room, 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.

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 the maps themselves, turning maps from simple, straight-forward walks, to seemingly endless mazes riddled with dead ends.

Controlling The Director
Using the console command sv_cheats 1, 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.

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 The Horde during a Panic Event, 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 awhile 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 at the exact same points in the map. Grenades, first aid kits, and the Horde are still controlled by the AI Director, though, and are used to balance the game if one team is dominating.