Valve's famous for play testing things to death -- were there any parts of Left4Dead that you thought would be a great idea at the time, but you then cut out completely?

CF: Yeah, we do that kind of stuff all the time. We always joke about these companies which have these design documents full of great ideas, and there are loads of things that sound really great on paper. They sound like The Winner, but then you go to test it and it's like -- oh my God, nobody knows what the hell they're doing, what's going on, it's horrible.

In fact, right now there are some things on these demo machines we're testing and we'll be getting feedback on and cutting and whatever. It's been interesting to watch some of these things with people who are good gamers too. Watching people who game at a higher level and seeing how they interact with things and what they do with it is always interesting.