Last week, I spent a day in Bellevue, Washington, at the offices of Valve Corporation. In four hours, I played through about half of the story mode of Left 4 Dead 2, the co-operative first-person-shooter video game about surviving a zombie apocalypse.

Mind you, the three others who were playing with me were people who developed the game. We were moving more quickly than the average person or group would. To say I was a liability to our group of four wouldn’t be an exaggeration.

While we were playing, Kim Swift, one of the brains behind the inspiring game Portal and a level designer on Left 4 Dead 2, explained the new co-op mode in the game, which will be released on November 17.

Realism Mode, said Swift, is for advanced players only, a way to make the game more interesting for players who don’t feel challenged by the expert difficulty level.

Turning on Realism turns off the glow that normally haloes players and special infected creatures. “Communication becomes really important,” said Swift. “Players need to stick together.”

The game is designed such that getting separated from the three other survivors is life-threatening. The glow normally helps players find their way back to the group, or helps find team members who may be incapacitated. Swift said that when players need help in Realism Mode, they need to be able to accurately and clearly describe where they are in the game environment so that the other players can locate them.

Health and items “don’t glow until you are right on top of them,” said Swift, which means that while regular gameplay might involve dashing from one safe area to another, in Realism Mode searching and exploration are essential to survival.

Swift said that Realism Mode also affects damage, so chest shots on the infected are not as effective, and players need to make head shots. “Just like in the movies,” added Valve’s Yasser Malaika as his Left 4 Dead 2 character killed a zombie that was about to finish off my character.

There are five campaigns in Left 4 Dead 2, each with multiple levels, and Swift said that all of them can be played in Realism Mode. And it’s possible to scale down the difficulty to easy in Realism, for those players that want to see what it would really be like to be in the midst of a zombie apocalypse, but don’t want to be killed too quickly.