It has often been said that the true star of Rockstar's groundbreaking Grand Theft Auto IV was not lead character Niko Bellic but, rather Liberty City itself, a fictional metropolis modeled after New York City. Creating a breathing, living city is one of the toughest challenges facing developers of open world games today: get it wrong, and your player feels like she's walking through a lifeless television set with cheap props and false fronts for buildings. Get it right, and she'll be absorbed enough in your world to momentarily forget she's playing a game at all. Ubisoft Montreal is among the best in the business at creating believable cities thanks to its Assassin's Creed series. We sat down with Alex Hutchinson, creative director of the upcoming Assassin's Creed III, for his tips on making a city feel alive. He tells us it's "one of the hardest things you could possibly do," but managed to offer the following.

1. Two-tiered reactions

It is a given that NPCs in open world cities will react to your actions: fire into a crowd in any Grand Theft Auto, and pedestrians will run screaming, cops will come after you, and some of the rougher types might even fire back. What Ubisoft Montreal has been focusing on for the Assassin's Creed is in two-tiered reactions: NPCs that understand not only the action the player has committed, but how it relates to them. Hutchinson explains: "If someone hires you to kill Character A, and you do it, and they react like, 'Good, that's what I asked you to do.' But then you go and kill some other random person and they're going to freak out, like you've lost your mind. Whereas someone who wasn't involved in that conversation would freak out at both of the instances." It sounds simple when explained that way, but Hutchinson insists that it is "one of those things that no one in games yet has truly solved," including his own team.

2. Random behavior from a pool

The Assassin's Creed series has perhaps the most believable crowds in games today: walk through an environment, and you'll see countless people going about their lives in realistic ways. The trick is that these NPCs are spawned semi-randomly. As pedestrians populate an area in the game, they are randomly assigned behavior from a large pool of possibilities, meaning it's unlikely you'll notice too many of them going through the exact same animation cycles. It's an expensive part of the series' development, Hutchinson admits, but it's a key ingredient in making a city feel alive.

3. Variety over complexity

Though a minority of your players will be troublemakers trying to break your game and peek behind the curtains (including this author), most of them will play your game just as intended. And in an open world environment, that means running quickly through the crowds from point A to point B. Take advantage of this! Most players will only look at your random NPCs for six seconds, max, Hutchinson tells us. So instead of focusing on a complex AI routine full of animation cycles that last much longer than that, put more emphasis on a wide variety of different, shorter actions.

4. City first, props later

Every Assassin's Creed to date has laid out its cities based on actual historical maps, worrying about filling it in with gameplay setpieces later. It's unlikely that your game will strictly adhere to an existing map, but it's a good takeaway: even a game like Assassin's Creed focuses on a believable city layout first and worries about making that city fun to play in later. Which brings us to our final tip:

5. Save your polish for mission areas

It's tempting to make your entire city a playground for explorers, and adding little touches will be rewarding, but remember that the places in your city where actual missions take place will give you the most bang for your buck. Have fun decorating the city and placing clever props where you can, but focus most of your energy on making the mission areas as fun as you can, as that's where players will be focusing the most attention.