Valve hasn't figured out yet whether the new Crash Course campaign will work with both Left 4 Dead and the upcoming Left 4 Dead 2.
"We're still struggling with how interoperability is going to work, right now," Faliszek told Edge. "When we talk about L4D, the story stuff - there's actually a lot of character dialogue that's based in each map."
Faliszek's concern is that the new Left 4 Dead 2 cast would not be anywhere near the locations from Left 4 Dead 1 - including Crash Course, which was the subject of discussion - which would make things a little awkward.
"We have a world sketched out and we have an understanding of where everything is and what's going on, you know, so story-wise it wouldn't make much sense," he explained. "But equally there's a whole group of the fans who don't care."
One option, he said, was to bring the original cast of characters along to the second game. "That's one of the routes we've talked about going, and maybe adding the new creatures, or whatever," he said. But the point is that it's not fixed, even with Crash Course out next month and Left 4 Dead 2 hitting in mid-November.
As for what is confirmed about Crash Course, Faliszek explained that the Versus focus was a reaction to feedback and statistics, and particularly the desire to reduce Versus sessions from two hours down to a sharp 20 or 30 minutes.
Crash Course is described as "a two-map campaign with a finale", and as discussed earlier this week it's set between the No Mercy and Death Toll campaigns, with Survival maps and the traditional option to play it as a co-op campaign.
The other big talking point around Crash Course was the decision to release it for free on PC while Xbox 360 owners are expected to pay 560 Microsoft Points (£4.76 / 6.72). We've asked about that but haven't heard back yet.