respect whedons' law
Quite simply? Don't be a dick. This is a game where there's a lot of room for characters to do horrible things to each other, but make sure that doesn't happen to the players.
Ooc communication is your friend
People have different limits when it comes to rp, make sure both you and the person you're threading with know what's up and respect each other boundaries. It also helps to have a list of what you are/aren't fine with on your characters profile.
Ic means Ic, even in Games
You may be fantastic at Cry Wolf, but your character may not be. Stick with being in character.
However, also respect the ic/ooc divide
What a character feels isn't the same as what the player feels, so don't take it out on them. In short, why do you have to be mad? It's only games.
When in doubt, warn about mature content
You don't want to be surprised by something disturbing, do you? Extend that same courtesy to your fellow player and warn in the subject line if something questionable is about to happen.
Be open to crit
Everyone makes mistakes, and everyone has room to improve. Be polite in giving critique, but also be okay getting it too.
In general just the basic stuff you see in any game! Let us know if you have any questions, or feel uncomfortable with a players' actions.