Its purpose is to enhance a player's "permission to believe" by providing an opportunity to enjoy an informed glimpse from behind the scenes and to develop an appreciation of the thoughts, ideas, technology and processes used to develop the game. Developer commentaries also include specific facts pertaining to the game's canon touch points; for example, one commentary states that Francis is six-feet five-inches tall.
After accessing the Developer Commentary mode, a player is invited to travel through a campaign chapter in perfect safety as all Infected will only attack the AI Bot Survivors. At intervals, there are icons called Commentary Nodes (refer the illustration opposite) which trigger a specific audio comment when the player presses the "use" key while looking at the icon.
Some Commentary Nodes do more than simply play a recorded narrative and go so far as to spawn objects, characters and even scripted events pertaining to a specific fact. In Left 4 Dead and Left 4 Dead 2, the Developer Commentary takes place in the campaigns No Mercy and The Parish respectively.
The following pages provide more in-depth coverage of the developer commentaries in each game: