Rockstar's Facebook app, Red Dead Redemption: Gunslingers, is not the kind of Facebook game you can get away with playing at work. While you could mute the sound and get one of those screen shields, the game is just too high-quality to cover up what you're doing.
At its core, Gunslingers is no different from other Facebook racing or fighting games -- you select an avatar, a class or type, and click on an opponent. Statistics determine who wins each match, doling out experience points or virtual currency to the winner. Rockstar, however, kicks it up a notch with fully voiced animations for each duel with separate (and bloody) animations for duel results. It makes it much harder to play the game passively and the bold visual style makes it impossible for screen-watching coworkers to look away.
So if you're sitting in an office somewhere reading this, click at your own risk!