Otherwise, in basic play, it's just the single-player game with friends. With a few changes. "We do scale up the enemies," says Heller. "That was really where we spent our time, making sure the AI scaled appropriately. So if you go in with four players on veteran [difficulty], it's a really hard game." The addition of the ability to revive fallen comrades - and failing of the level if you don't - completes the basic experience.
But there are other approaches to co-op to bear in mind, like competitive co-op. This is where you play with your friends in the same levels, and compete for scores. "Almost like Geometry Wars, you get multipliers as you get more kills, until you get the ten-times multiplier. And to get to nine and ten times? It's melee kills only. So you can get a score in the millions when your friends are sitting around in the thousands." The obvious comparison is Bizarre Creations' The Club, except with multiple people playing through a level trying to max out their score at once. Yet again, memory plays a large part here, in knowing exactly when to expect enemies to appear so you can get them first.
While it immediately shatters the game's credible atmosphere, it replaces it with something else that seems to be as compelling. "There's a really cool tension, because you're competing and trying to run through the level as fast as possible... but if you die, one of your friends has to revive you, or you all have to go back to the last checkpoint," says Heller. "So if someone dies, what you normally do is dive in there, get all the kills that you can and then revive them, which is worth a score and keeps your multiplier, but you've taken all the kills and he's taken a real penalty."
That's not all, apparently. "There's a secret little co-op thing that'll be unlocked at the end of the game and we're not talking about it yet," Heller smiles. At which point I mention that I saw the word "zombie" on a mode select menu when a game was being demoed to me previously. He laughs. "I believe the word has been leaked, but I can't be on the record about it," he says. "I will say though that it's awesome enough that it's what we play all day. When we have a break, we play that." (Since Heller spoke to us, Treyarch has confirmed that you and up to three friends will be able to fight off zombies, and buy weapons and other defences between waves.)
So what was on the team's mind when it was doing co-op? What was its purpose? "We wanted it to be a gateway to multiplayer," says Heller. "The conventional rule of thumb for Call of Duty is that single-player and multiplayer players are mostly separate. We have players who play in both camps, but they don't really cross over as much as you'd think they would. We think that co-op could be the place for them to meet and have shared experiences."
Equally, that's not just a case of playing together - it's setting you up for a more traditionally competitive multiplayer experience. "When you play co-op there's challenges you can unlock to earn experience, so you can actually enter multiplayer maybe level 10, maybe level 20, without ever playing a multiplayer match. And that's a good way to get started which isn't intimidating for people."
Call of Duty: World at War is due out for Xbox 360, PS3, Wii, PC, DS and PS2 on 14th November.