"Counter Strike was multiplayer only with five minute rounds in which you can be killed in the first five seconds and spend the next four minutes and fifty-five seconds sitting and waiting to get back in. In a similar fashion, Left 4 Dead is being completely unapologetic about saying 'if you don't play as a team, you're not gonna make it'.
"It's really about beating people into submission to make them play as a team - to play cooperatively," he adds. Alone, even a single infected can kill you. Hunter zombies leap huge distances, pinning survivors to the ground where only a team mate can rescue them. Wounded survivors fall to the floor, immobile until a friend pulls them to their feet. Dead players don't turn infected, but can be found later in each campaign, sealed in locked rooms until released. Healing takes ten seconds or more and survivors carry only one medikit which can be shared with a wounded comrade if necessary. Better four guns than three, right?
As we chat with Doug, a scream - half shock and half excitement - goes up from four European journos knee-deep in the Hospital campaign. "That," says Doug. "That's what we're trying for, right there. When you play Counter Strike and you just manage to win and you have that watercooler talk about all the things you did afterwards - those are the best gameplay experiences that we have. To get that same high that you get from that perfect Counter Strike game? That's the goal."
Primarily a cooperative game, Left 4 Dead has its own version of the Deathmatch too; a mode which has inadvertently become the perfect counter to the team-killing idiots who threaten to ruin any teamwork-centric game.
Doug explains: "The multiplayer mode allows you to play as the boss infected, and that to me is a griefer's paradise. Your whole thing is to go and cause chaos. Griefers are gonna be the best boss infected to play against, right? They're already causing chaos!" Like the survivors, boss infected come in fours - blood vomiting, morbidly obese Boomers; agile Hunters, climbing and leaping like Spider-Man; Smokers, with a whip-like disturb-o-tongue; and the colossal Hulk-like Tank.
Fighting alongside the regular infected, the boss infected can see players through walls, allowing them to stalk the survivors from afar. No one boss infected is a match for the four survivors in a straight fight, but in the right place at the right time alongside the infected army, they're a nightmare.
The fifth, unplayable boss infected is the game's most terrifying horror. Fleeing through the industrial complex we run across elevated beams and catch sight of a Witch through a floor grating. Only seen once per campaign, she stands crying in the darkness, all pale skin and matted black hair, her agonised moans warning you to extinguish your lights and aim carefully. As strong as the Tank and as fast as the Hunter, she can kill instantly but only becomes agitated when disturbed by gunfire or flashlights.
You become quieter when the Witch is around; you tense up and try not to look her in the eye lest a twitch of your finger sends her into a screaming rage. It's that tension which makes the game - a shared tension, alleviated by jokes between you and the other survivors, punctuated by moments of extreme action and all-too-brief periods of calm. At its most thoughtful, Left 4 Dead is a true survival horror game, and at its most frenzied, it's the fastest and most intense first-person shooter you'll probably ever play.
Consoles are up to the job of survival horror, but that speed is where pad controls just can't match a mouse and keyboard where a full 180 is just a flick of the wrist and a set of hotkeys allow instant access to weapons and equipment. "That's one of the things we'll be working on over the next six months," says Doug, when poked. "We're pretty proud of Half-Life 2 on the Xbox and the 360, and Portal, Episode 2 and Team Fortress 2 are different games which were great experiences on both platforms.