It's a careful balance between scripted events and on-the-fly procedural moments generated by the game's Director AI system, but it functions smoothly and prevents the action from slipping too far in either direction. Faliszek went on to explain that while the Director system is still subject to a great deal of refinement, it's proven immensely successful in Valve's testing phases.

"We're always tweaking [the director], but it always had the good effect of seeing people playing through twice -- they're so trained to think that what happened last time will happen to them this time, and that always comes back and bites them. It's always around the third or fourth time that they realize that you can't really count on the action happening that way. We've actually gone so far, in the newer version, where the director determines what the placement of items are, so they're spread out a little more randomly. It's meant for replayability, so that you can never rely on these sorts of events."

My second playthrough of the urban scenario was most certainly different from our first, and not just because of the engine updates and other additions. Boomers and other boss zombies were appearing in different locations than I had remembered, certain rooms more populated than others had previously been.

But the witch is perhaps the most destructive agent of unpredictability in Left 4 Dead--run too quickly, don't check your corners, and you're bound to stumble upon her in a most unfortunate location. We saw her again, later in the scenario's decrepit, aging hospital level, saddled smack in the middle of a hallway intersection. Chet described it as a "bad spot," as it ensured that we'd have to take her on rather than simply sneaking by.

"On the street we took her out there, pretty good location," he said. "We had a lot of space, we could throw the fire at her. But there are times when you hear her and you're kinda creeping along, and some other action happens and you're running around a corner and all of a sudden there's the witch, and you set her off."

I asked him what exactly "sets off" the witch--knowing full well what she could do, I was in desperate need of information. It would've been a total drag to get us all killed by blinking or speaking aloud or breathing too hard, after all.

"Aggravating her. Getting too close, shooting near her, looking at her with the flash light—you want to make sure your flashlight's off," Faliszek explained. "You pretty much want to leave her the hell alone."

Chet asked Valve level designer Scott Dalton, one of our in-game comrades, to set her off, suggesting that it was fun to watch him die. Dalton hit her point-blank with a shotgun round and was almost immediately mortally wounded in the flurry of attacks that followed. The witch is lightning quick and does a tremendous amount of damage in a short amount of time, perhaps even more than the hulking Tank boss infected.

Valve and Turtle Rock have made a good deal of progress shaping Left 4 Dead into a title worthy of its pedigree, and the prognosis is looking great. It's still some of the most fun time I've spent behind a mouse and keyboard, and even on a second playthrough of a level, the tension and anxious atmosphere were as potent as ever.

Those put off by the recently revealed November release date for the game can rest assured that the time will be well spent. As Faylor and I saw last January, the game's already in a great place. I stick by my first assessment: with some tweaking, testing and a bit of polish, Left 4 Dead could stand as one of the best multiplayer PC efforts seen in years.