The exact algorithms are, of course, a mystery; all I can pass on is that in its current state, it gives you an excellent sine wave of stress, and is - yes, yes I know - as much like a bloody film as any game without cutscenes, a script or sweeping aerial shots of Cardiff can be.
The music is an important part of that film-like experience, and smart players will take cues from the heralds. The Tank has a particularly dramatic theme tune that warns everyone that the game's superpowered bullet sponge is on the way.
Rather than ruining the surprise (how much fun would that surprise be, anyway?) it gives you and your team a chance to organise, to regroup, and to focus your firepower. The Tank will screw you over unless you're all working together to chip away at his hit points.
Behind the music is Mike Morasky, a prolific musical genius whose career spans many groups and styles. However, there's a golden thread of churning atmosphere running through much of his back-catalogue.
Play Left 4 Dead while listening to Milk Cult's Love God album, for example, and the journey back to inner peace will be a difficult one, characterised mainly by scratching your own forearms and glancing at the doors. His L4D score is less experimental, but thanks to the clues it gives you about what might be around the next corner, it's plugged just as directly into your emotional response centres.
While we're on the subject of prolific and uncomfortable musical legends, the experimentalist Mike Patton voiced the Infected. Portal fans with a love of Patton's side projects may also be aware that he voiced the gibbering of GLaDOS's final, angry orb. Cool? Not much.
"Turn your flashlight off! Don't look at the witch!", commands Chet. The Witch is the game's Ring piece, a sobbing girl who only surfaces from her lachrymose reveries to rip you apart with one hit.
For that reason, it's considered prudent not to shine your torch on her, or shoot her. I reply that I want to look at the Witch. I'm supposed to be writing about the Witch, and I'd look like a proper 'nana if I came back from Seattle to write "I didn't want to look at the Witch because she was a bit scary". So I snuck a crafty sideways glance at the slouchy mare, just as a teammate let rip with a volley of machine gun fire.
I'm not sure if this was simply to show off her powers, but it certainly had that effect. Sobs became a crescendo of screams, and the inert girl who wouldn't have looked out of place keeping a YouTube vlog began killing us. The guy who'd shot the gun was the first to go, but she came after me straight after, perhaps in payback for my crafty glancing.
Everyone else had run off by this stage, leaving me with about 20 seconds of bleed-out time to stare at the now-purposeless Witch, who resumed a kind of hopeless, agonised writhing which would have looked pitiful if she hadn't just butchered me.
Eventually, players will be able to play as the Super-Infected, but that's not being shown to critical journalists at this point. In dead mode, however, I did get to see what it might be like; letting my camera follow a Boomer, I saw whole areas of the map invisible to the living players, with short cuts to facilitate ambushes - even a couple of inert regular infected zombies.
Portal's Kim Swift mentioned that she enjoyed playing the Infected most of all, and it looks like it might be slightly more challenging. While there's less of a definitive sense of victory, it's sure to offer plenty of big dumb fun for people who love to ruin other people's games.
It feels like I could write forever; but time, word limits, and the scowls of a production editor who's married a deadline mean I have to stop now. Do I think I've managed to get across the experience of playing the game? Of course not.
The best I can hope for is to get across how much I enjoyed it - and try to infect you with my impatience for the final game's release.