How do you play as the Original L4D Survivors?

[TheWinter]

Hi there. I am just very confused, because I've seen several videos of Left 4 Dead where the people play as the original Survivors from the first game and either I'm just extremely stupid, or there is some kinda condition that has to be met for this. So, I just wanted to know what you have to do to play as the original cast.

[IDLE: Center]

You need to play L4D1 Maps in order to play as the original survivor

[JAiZ]

By default, the Left 4 Dead 1 campaigns are the ones that will feature the L4D1 survivors.

Those campaigns are as follows:
No Mercy, Crash Course, Death Toll, Dead Air, Blood Harvest, and The Sacrifice.

There are also various custom campaigns that feature the L4D1 survivors as well.

If you want to play as the L4D1 survivors in the L4D2 maps, refer to the comment above.