A lot of work has been put into the co-op. Lead Designer Colin Munson says “Eight players is that sweet team size. It’s enough so that you pretty much know the people on your team but it’s big enough to let new players in.” The co-op campaign will focus on teamwork, with the game testing players ability to work together. “The initial bridge attack forces players to deal with danger from multiple angles. The Titans meanswhile require the squad to divide and conquer, as these behomoths can easily take out an entire squad with one hit from a rocket cannon.”
The co-op mode also allows players to choose a character class. These are soldier, medic and special ops trooper. As you progress through the game you'll gain XP, and then can level up that character with new weapons and abilities, much like Call of Duty 4's multiplayer.
There's also a video up on ps3.qj.net with Insominiac President Ted Price.