In Left 4 Dead, the players come across a church in the Death Toll campaign. Once they get there, they find a safehouse. The only problem is that a maniac is inside arguing with himself, and he refuses to let anyone in. If the players listen for a while, they will learn that he was bitten by the last person he let in. The player that talks to him will often say something humorous, such as Louis threatening him ("I will come in there and beat you to death with my gun!") Talking to him will cause him to lose it and ring the bell, alerting hundreds of zombies.
Once they are all dead, the Survivors open up the door. The man will have turned into a random Special Infected at this point and will attack the players. Inside the saferoom, the Survivors can find words written all over the wall, such as his motto "Better safe than sorry" which he muttered over and over again to himself shortly before his well-earned death.
Church Guy is voiced by Nathan Vetterlein, who also did the voice work for the Scout in Team Fortress 2.