How do you follow up one of the most successful video games of this generation -- a mere year later, when it's still selling for full price? Activision senior producer Noah R. Heller is confident that Treyarch's latest iteration of the Infinity Ward-originated Call of Duty series, World at War, is up to the task.

Here, Heller discusses the technical and gameplay innovations that drove the development of the game at the Activision-owned Treyarch, which previously created Call Of Duty 3 and has just completed Spider-Man: Web Of Shadows and Quantum Of Solace, the latter of which also uses Infinity Ward's engine.

In this in-depth Gamasutra interview, Heller talks about working with Infinity Ward's engine technology, the collaboration between the two studios on the series, and the pointed mayhem that may become the hallmark of World at War -- both from an artistic and technical perspective.

You've used the Call Of Duty 4 engine for your game and, owing to that title's popularity, I'm sure you've incorporated many of the mechanics that made it such a huge success. But could you elaborate on those areas where you've struck out on your own in terms of design?

Noah R. Heller: Probably the biggest innovations revolve around the flamethrower. We built a really good fire system from the ground up. It does a lot of interesting things, propagating through the environment based on an actual wind vector.

So if you spray fire at the edge of a field, you can see it spread through the field, travel up a tree, and burn out the sniper perched in its branches. The entire environment is flammable. It's not a destructible game, but enough gets set on fire that we think it makes for really great interactive options.

The other thing we had to work hard on was communicating the Japanese military. Call of Duty has always been a pretty tightly-scripted experience, so we had to go a lot farther than we have done previously to create a Japanese enemy that was realistic.

They're much more aggressive. They have the ability to climb trees, to hide in foliage, to jump out of spiderholes, to ambush you, and to basically wait for their moment to pounce. They also have a disregard for their own well-being, which is a little bit different for Call of Duty.

Then, when it comes to cooperative play, we built the entire game so that the single player missions were also available as cooperative missions. That meant the levels needed to be bigger and feature alternate paths.

But we didn't want to take the soul of the levels away. Call of Duty is about you, with your buddies, trying to just get over that hill, trying to get to that objective. We didn't want something like: "Okay, you stand here and set the detonation charge and you stand over here and blow the charge." We wanted that gameplay to kind of evolve naturally, and that took a lot of environment work and crafting.

Then, on the multiplayer side, we really wanted to include vehicles. Treyarch has a long history of doing vehicles with Call of Duty 3 and United Offensive... We limited ourselves just to tanks because we wanted to get it perfect. [Ed.: United Offensive was developed by Grey Matter Interactive, some members of which were integrated into the Treyarch team following United Offensive's release and the studio's acquisition by Treyarch parent Activision.]

We didn't want to mess around with vehicles that might not work out. We built an armor system for the tank, so the tanks have detachable pieces of armor that you can blow off. And then, once you've detached that piece of armor, you can throw a sticky grenade in there and do extra damage.

Basically, wherever we could find a system where we felt like rewriting it from scratch, we did, in order to improve the engine.

So, on the point about it being quite scripted...

NH: And it's still scripted for this one. We had to build new and more intelligent scripts.

But one of the things that frustrated me with the previous game was the invisible checkpoints with enemies, where they'll just keep respawning until you pass an arbitrary, hidden line. Have you reworked that system?

NH: I can't say that there's no place in the entire game where enemies might be infinite, but believe me, it was a big concern to us. I'm pretty sure we reduced it down to nil in almost any situation. Except one, where from a plot perspective, it was important for enemies to keep pouring out to get the player to move on.

A lot of people are completists, and they want to kill every enemy before moving on, so we wanted to make sure that there was something for those players as well. Basically, we get it; we get that it's bad gameplay when you show how the engine works to people.

So anywhere that occurred, we tried to reduce it or limit it completely. I can't promise you you're not going to see it, but you're not going to see the clown car effect where thousands of enemies just keep streaming out of a house or something.