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 AI Director, simply Director, or 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 each playthrough. 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.

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.

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 the like. When running a dedicated server, the administrator of that server can create a controlled environment by completely turning the Director off.

Reading the AI Director
With a bit of practice, it is possible to anticipate moves by the AI Director. For example, if an area is full of Common Infected, you can expect the director to spawn some special Infected to harass you while you deal with The Horde. If you pass multiple areas with very few Infected, it is likely you will be dealing with a Witch or Tank soon. The AID is also easy to read in Versus mode because the Director tries to be somewhat fair to each team. After the January 14th patch, Tanks and witches for both teams spawn at the exact same points in the map, though grenades, healthpacks, and the Horde is still controlled by the AI Director and will try to balance the game if one team is dominating. 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.