Keep it civil, do not flame or bait other users. If you notice anything illegal or inappropriate being discussed, contact an administrator or moderator.
Either I forgot or don't know where this rule is recorded (I looked for it in the sticked FAQs and the TOS but no luck), but I know for sure RP is not allowed here. Just telling you before a mod does, as you're not the first person to ask.
Edit: oh, I see it says so clearly whenever you're about to post a comment on the image board... I guess if it applies there then it also applies in the forums