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THE REAL DEAL
Survive a campaign on Expert skill with Realism mode enabled.

"Realism Mode", eh?...

In fact, there are quite a few curious things that these achievements bring up.

Using a "bile bomb" to kill a Tank gets you one.
You can honk the noses of Clown Zombies.
The defibrillator is confirmed. It revives dead teammates.
The Gnome from Half Life 2 makes an appearance. Leading him throughout the campaign Dark Carnival gets you an achievement.
You can collect vials of Boomer Vomit from CEDA agents.

Quite interesting...

What exactly is realism mode.

Ok, got more data on Realism Mode.

First change in Realism Mode:

Glows.

You know the glow that surrounds your fellow team mates? Realism Mode makes it go BYE BYE. You get lost? Talk to your team so they can rescue you. Pinned by a Hunter, while vomited by a Boomer? Good luck surviving THAT without calling to your team and telling them to follow the infected mob, because that's the only way they'll find you.

Or the glow that surrounds items and ammo at a certain distance? Well, cut that distance from METERS to CENTIMETERS and there you go. Good luck finding those incendiary bullets that are out of the way in a dark place near a Witch, now you'll really need to turn on those lights. No following the glow, here. Again, talk to your team mates if you find something, and tell them where the hell are you. No glow = exploration must be done in teams with high communication.

Second change in Realism Mode:

Damage.

But not to the damage survivors take. The damage they give to the zombies. Zombies are tougher, more resistant to your shots. Unless you've been watching too many zombie movies, then you know what to do: Shoot them in the head. SHOOT. THEM. IN. THE. HEAD.

That is all.

I don't think these are true. FF damage depends entirely on the game's difficulty. The article I read mentioned that you can plan Realism Mode in the Easy difficulty. I don't think Realism Easy will have the effect you mention, although I wouldn't be surprised if FF damage was altered to make headshots far more damaging.

We have Avatars as in, little custom square pictures.

Playing Realism Mode.

The only out of place comment is the one about the Witches. The incredibly high number of witches is a feature of the Sugarmill, part of the Hard Rain campaign, not of Realism Mode. This has been featured elsewhere.

Also of note:
You can't even see your team mates name tags on top of their characters, which means it can be REALLY EASY to mistake them for a zombie if you're not alert and careful.
Apparently, there are no rescue closets in this mode.

Wut nao?