Oh, also... when you come out of the tunnel of love, a clown will lead a whole horde over a wall...

The artificial intelligence of Left 4 Dead features a dynamic system for game dramatics, pacing and difficulty called the "Director". Instead of fixed spawn points for enemies, the Director places enemies and items in varying positions and quantities based upon each player's current situation, status, skill and location, creating a new experience for each playthrough.[6] The Director also creates mood and tension with emotional cues, such as visual effects, dynamic music, and character communication.[7].

Valve has termed this dynamic set-up "procedural narrative".[21] In addition to the AI Director, there is a second Director that controls music. It was created as a way to keep the music interesting throughout the game. The music Director monitors what a player has experienced to create an appropriate mix. The process is client-side and done by a multi-track system. Each player hears their own mix, which is being generated as they play through the game, and dead players watching a teammate hear their teammates' mix.[22]