1. Understand and respect the rules.
Every roleplay has rules. Sure, they may take the direct route out of a situation sometimes, but the fun comes from using the limitations in creative manners to get to your intended destination. Plus it stops everyone from doing pretty insane things (which I'll talk about later).
There is usually a list in the first post about what will and will not be tolerated. Read this, it will save you a lot of grief. But there are also a lot of "unwritten rules", like etiquette and...well, essentially what this post is all about. So I suggest you bone up on them too.
Oh yeah, did I remember to state how important this is? No? Well, it is very important. On this forum, when we RP, if you disrespect the rules, we will tell you. Usually this gets everyone back into line again, and things carry on as normal. On the NS forum, however, it can get a little more heavy, as they have moderators who are specifically there to keep things running smoothly. (Yes, our staff here do the same, but we participate as much as everyone else here, we're a little easier on you guys, plus we don't have our own little staff cave where we hold stuff akin to the Mod Olympics.) And given the sheer size of the game, they can afford to come down a little harder on rule breakers. That means bans. And getting banned not only from an RP but from the entire game, for however long it is, is not a good look.
2. Do your research.
This is also very important, from multiple standpoints. Know about the roleplay that you're participating in. The signup thread usually gives a rundown on what's happening, so you know what's going on. The first post also sets the scene for you. Read these intently, as it'll save you a lot of grief and embarrassment.
Also, make sure you have a working knowledge of the sciences such as mathematics, physics, chemistry, biology and technology, as well as anything else you may need to know. People will give you grief for mixing up basic information. A simple rule of thumb is, "if you don't know how it works, don't include it."
Get to know your fellow players' characters. This bit of research is both an in-character and out-of-character exercise, and builds a lot of rapport with your fellow players, as it means you're willing to participate and cooperate, even if you're eventually going to stab them in the back. (No, not everyone does that, but you get where I'm going.)
Finally—and this last one is recommended, but not mandatory—get to know about something called tropes. These are storytelling devices, and since roleplaying is essentially telling a story, getting to know these devices can help you out when it comes to playing.
3. Allow some character development.
"No, my character wouldn't get herself into that kind of situation. I refuse to play any further until this is rectified." I haven't heard anyone say this exactly, but there are some players out there who seem to know everything about their character, and hence do not allow their character to grow. A big problem with that: what if they're in a situation that is outside their experience/your consideration? Character development is what allows your character(s) to grow, and also give you an opportunity to grow as a writer, as every new situation provides you with a new chance to get creative.
It is possible that characters will enter familiar situations, but usually something is a little different the second time around, which means a modified solution (and another chance to get creative).
Speaking of characters...
4. Don't play someone else's character.
It's a respect thing. They created the character, so do not assume you know their reactions to what you say or do, because they will surprise you.
On a more general note here, though, respect your fellow players. Insulting them in-character is okay if the plot warrants it; go overboard, or do it out of character, and whatever respect you earned will be lost immediately.
5. Don't godmode.
This kind of combines points two through four, and is generally included the roleplay rules, but I'm going to list it here, because this is a very important thing to remember, and a very easy mistake to make on your first time. I said in point three that you don't know everything about your character; the same extends to your character's knowledge. You don't know everything, you can't do everything, and you can't be everywhere; neither does/can your character. Godmoding tends to break the game, and stop making it fun for everyone else. Keep your characters—and your gameplay in general—grounded and realistic.
Allow me to give you an example:
I have a character, Gempidon Deach, who acts as a sort of agent on orders solely from the Emperor (and never a third party). Now, say she had been asked (however unlikely it is) to assassinate an influential citizen of another nation. Sniper-style, for the sake of this example. So she sets herself up and takes the shot.
Now, he could avoid the shot. How is up to the player, and the direction he chooses will determine whether he godmodes or not. A person walking in front of the shot is acceptable, albeit unlikely in real life (it does, however, happen). Him spotting her without any visual aids and dodging the shot of his own volition is not acceptable, unless he is a cyborg, in which case it would've been revealed earlier. That second one is godmoding.
As I said before, godmoding breaks the game, and takes the fun out of the roleplay. I've been in an RP where the OP godmoded, and believe me, it's no fun when everything you do ends up with the ruler of the nation completely obliterating your cast. (Many of the senior region members have too been in situations where godmoding has occurred, and their experiences are exactly the same.) Moderators on NS don't take too kindly to godmoders, and tend to deal with them very quickly. We don't like them here either, although we tend to go more towards ignoring those posts (and, in rare cases, suppressing godmode posts, although at the time of writing, we had managed not to resort to that). Even if you don't get caught, you realise very quickly that continuously playing the god means you lose a lot of players from your game, and it gets very boring and very "unfun" very quickly, because no-one wants to play with someone who can essentially nullify everything they do.
There is, however, a way to avoid that...
6. Don't be afraid to ask for help.
The main NS game has something called "mentors", who are essentially highly experienced roleplayers who have been recognised for their contributions. Because of their experience in the RP world, they know a lot of what makes a quality roleplay post. Don't be afraid to ask them questions about how to roleplay, because they may very well have the answer to that bugging query.
Here on the regional board, we don't really have mentors; that job falls mostly to our staff. But we are still willing to answer questions about roleplay, including any concerns you have about how you play something.
In either case, make sure you take what is suggested to heart; we aren't just throwing around random stuff here. We are helping you out here, so having our advice completely ignored can lead to...well, I've already spoken about it.
7. Don't gatecrash.
Have you ever been to a party when someone who wasn't on the guest list turns up and starts wrecking the atmosphere? Pretty terrible, isn't it? Same feeling occurs when someone gatecrashes an RP. It throws everything out, because you have this new unknown come in, and it's especially bad if they start posting from the get-go, completely nullifying everything you've worked towards.
Many RPs have a signup thread for this very reason. A few other RPs are invitation-only, which means you need to ask to join before you can post. Only a very minute number of RPs (less than 0.1%, by approximations) are completely open.
8. Have fun.
This should go without saying, but this is why we RP. If it becomes a chore to RP, then you need to take a break from it.