You don’t survive the apocalypse by being stupid. PlayStation 3 exclusive The Last of Us, scheduled for release some time in 2013, takes place over a decade after an outbreak that turned part of the population into hideous monsters, prompting a total societal collapse. Nature has reclaimed broken cities, and the remains of humanity eek out an existence with whatever resources they can scavenge, make, or steal. Natural selection takes care of anyone making foolish decisions.

So developer Naughty Dog populated its post-apocalyptia with smart people who react to what they see and hear like real people might. That builds a realism and immediacy that, as demoed this year at the Electronic Entertainment Expo, no other game can match.

It also makes things much more dangerous for the two lead characters, grizzled, player-controlled Joel and his charge, 14-year-old Ellie. While attempting to sneak through a building without drawing attention from the men scavenging it, you don’t just have to worry about one lone adversary at a time, but all of them.

“The A.I., from Ellie to the humans we’re creating, is a huge undertaking,” says Game Director Bruce Straley. Those scavengers? “They’re working as a group, and they’re aware of each other.” When Joel throws a bottle, it creates a predictable video-game distraction that one scavenger investigates … and one of his buddies checks up on him. “One of them’s concerned about the other’s welfare. ‘Are you all right?'” It’s not a pre-scripted sequence.

Likewise, as Joel and Ellie roam this world, they might have conversations based on things they see, and often call out to each other to make sure they’re OK while out of each other’s line of sight. Movie posters for Dawn of the Wolf (a riff on the Twilight series) might prompt an exchange if you indulge Ellie’s curiosity.

In a more dangerous situation, however, every single dynamic takes on major significance. When they’re discovered, Joel pulls out his revolver. Instantly, a scavenger warns his friends: “Aw, shit! He’s got a gun!”

“How many did you see?”

“Only saw one! He’s right by the door!” And yes, that’s right where Joel’s hiding. When he fires his gun, Ellie jumps, covering her ears. One of the scavengers gets the jump on Joel, bashing him up and adding a nasty cut across Joel’s nose before putting him in a choke hold for the men aiming from the windows. They take their time, making sure they hit Joel and not their friend.

At times like these, we’ve seen Ellie jump in before, using her butterfly knife to stick someone who’s got Joel on the ropes. This time, nothing … because those gunmen at the windows will take her out if she tries, and she knows it. Naughty Dog built a survival instinct into Ellie. It’s so good, they decided that players didn’t need the ability to direct her. Ellie directs herself.

Everyone else got those instincts, too. Joel breaks free and runs, quickly looting a gun off a downed enemy — a scavenger bursts into the room behind him, sees the gun and, lacking one himself, immediately turns and runs. The player controlling Joel decides to give chase, and Joel checks every corner for an ambush, muttering “Where the hell’d you go?” under his breath when he loses his prey. A noise behind him: His target’s got a molotov, but a snap-shot from Joel throws off the scavenger’s aim; the bottle crashes into the wall, setting it on fire. Desperate, the scavenger rushes Joel, screaming, and takes two bullets in the chest, dropping in a heap.

Ellie catches up, and stares at the widening pool of blood.”Oh, God,” she breathes.

“Let’s keep moving,” says Joel.

And none of that was pre-scripted, either. Each event dictated what happened next. If a player took a different path, none of this would’ve happened.

That said, developer Naughty Dog won’t divert from its brand of linear storytelling. Drawing enemies’ attention to Ellie’s location — effectively using her as bait — won’t make her trust Joel any less. “We think a good story is a story on rails,” says Straley. He trusts that, like the characters in his game, players will react appropriately to what they see and hear. “If we do our jobs, you’ll feel the same way the characters feel, and you’re motivated and aligned with that.”

Just don’t forget that the other guys are motivated and aligned as well.