In The Last of Us, players control Joel who has seen the world fall apart after a "fungapocalypse" while Ellie was born after modern civilization crumbled. Ellie is completely controlled by the computer, but The Last of Us won’t be one long escort mission. Ellie has been designed to react to situations and she will grow throughout the game. In this interview, Bruce Straley, Game Director, explains how Naughty Dog developed Ellie and the enemy artificial intelligence.

PART 1: On Blending Gameplay And Story Together In The Last Of Us

While players won’t be able to directly control Ellie, what kind of choices will the AI get? Will she develop over time and learn new abilities?

Bruce Straley, Game Director: It would be a really flat story if only one of the characters’ arcs and moves and characterization developed. Ellie needs to move somewhere, too. Even though she’s capable from the beginning and comes from a dark world, and has done some things that show she can exist on her own. She’s not used to the world and stuff Joel has to do to survive.

You’re going to see her working with Joel to learn more and more capabilities. Like, she starts not knowing how to use a gun, maybe. But maybe she can pick up the gun and Joel can show her how to use the gun, and she’s going to acquire more and more of these abilities as she goes [through the game]. She’ll gain more confidence with this as well.

Can you tell us how you developed the AI for Ellie? It seems like the AI is really competent for a partner character. With respect to gameplay, Ellie will develop as she grows as a “person” when Joel teaches her how to survive.

It’s interesting that those two things actually correlate. Just the same way Ellie is growing as a character, she’s becoming more and more trusting of Joel, [and] more capable. She’s gaining skills like the usage of guns, scavenging the environment to help Joel and find things. She has a backpack, so she can store items.

About the AI, in Uncharted 2, one of the things we started on from the beginning of the project [was that] we knew we wanted to have these really big set-piece moments, like the moving train and we wanted a building to collapse. So, we wanted these moments, and from the very start of the project, we started working on the technology to allow us to do that. Well, the same happens for The Last of Us with the AI.

From the start of the game, we knew, to personalize these characters; to make sure they feel human and are they are making intelligent decisions; [that] they’re not getting in the player’s way and it doesn’t feel like a babysitting mission, that was the first thing we took on. We’ve stripped down our AI, we’ve torn it apart and reworked and reworked and reworked. We’ll continue to rework it.

The last thing we got done in Uncharted 2 tech was probably that moving foreground object stuff. The local and global pairing of objects to allow us to do the moving train, etcetera. That’s the exact same thing that’s happening now. We’re pushing and pushing and pushing AI, and it’ll continue to evolve until the day we ship to make these characters real.

What about enemy AI? Are enemies going to be as reactive?

Again, we’re trying to make these feel like humans. So, this means the animations have to change. This means the behavior and decision making has to change. There’s a lot of attention to detail. If I pull out a gun and shoot it, how is this group of people [here] going to react? They would probably scatter. And then if they’re in survival mode, and they know I have stuff that they want, how many are going to group together and try to take me down?

It’s interesting to look at humanistic behavior. The scariest human is one backed into a corner. So, when I show my gun and when I start shooting, that’s going to cause a greatly different behavioral change [in enemies] than if I came out with a melee weapon. If there’s four guys vs. two guys, it’s very different than if there’s only one guy by himself.

We’ve thought about all of these different scenarios and we’re modeling in all the different, most practical behavioral choices that a human, in a fun gameplay way, would make. So, it’s still provocative. If there’s one guy and I have a gun, of course, he’s going to go hide. That makes sense for that guy. Now, whether he’s hiding to ambush me or hiding to get away from me, that might change as you go through the game, depending on the environment you’re in or the types of people that you’re around.

Can you tell us what your favorite unique weapon is? What’s a really unusual weapon or item?

Well, smoke bombs are pretty interesting. We have something in the AI called “dynamic stealth”. Even though the enemies have seen you and started combat, once I run away and hide and I’ve broken the line of sight, they’re gonna go searching for me. Then it turns back into, if you want to try sneaking around them, you can. This all happens dynamically. It doesn’t mean you have to keep fighting the whole time.

The smoke bomb is a great way to break this line of sight, and to go somewhere they don’t expect. The smoke bomb also creates a small concussive burst, so it allows you to rush them as well. Everything has multiple uses, so it’s not as much the uniqueness of the weapon as it is all the different ways you see the AI can react to them. That brick, that I could use to bash you in the face, or throw it, or throw it at the environment.

How much of the world will we get to see? How far will Joel and Ellie get to travel? We saw they were looking for a car in the video, so how much of America will they get to see as they travel West?

We don’t have a vehicle sequence—a driving sequence—per se, but we have vehicles that you can procure. Bill’s the scene that you saw here at Comic-Con. Bill knows where he can get stuff to get a truck back up and running. So that truck is going to be used by Joel and Ellie to get further faster than if they were just going by foot. So, yeah, there’ll be different vehicles—different means of transportation—that you’ll find throughout the journey.

The journey goes from the east coast to the west coast, so yeah, they’ll make it really far over the course of about a year.

We don’t need the vehicle to be drivable. We just need the vehicle to get the story purposes across. You’re going to be able to see multiple environments quicker through this game if you get a vehicle and it gives Joel and Ellie a great goal to find one.

Since The Last of Us goes through a year, will you have time skips in between?

Yeah, we’re going to have season changes and time skips and collapsed time, etc. To, in a cinematic way, feel like you’re compressing this journey into a palpable timeframe.

What’s your favorite point in the journey?

I probably can’t say it right now. It would be spoiling the game. To tell you the truth, the AI, the interactions with the enemies, and the choices that you have, whether I’ve crafted a Molotov or smoke bomb or some other inventory item, and used it, and seen how the enemies, the humans, react it—that’s special to me. And to see that these humans feel like they’re making intelligent choices, that’s great.

And of course, there’s moments with Joel and Ellie that literally kind of bring a tear to my eye that happen throughout the story. They’re beautiful moments that I can’t tell you anything about, but I’m really excited to reveal at some point.