How should you roleplay in Nimue? Be respectful of others above all (in terms of offending or intentionally hurting someone...even evil characters need to operate within the parameters of good taste), but after that it is whatever you and you character have decided how to act. Playing a zealous hero who has chosen shield mastery because all he wants to do in life is protect others? Say so, act it out, tell that little magician you just saved that you are just doing your job. Emotes are a blessing to a game that cuts out inflections in the voice (and therefore all subtle forms of interaction, such as sarcasm, hatred, joy, etc.) and use them often to convey what you are feeling.
How should you roleplay in Pantheon? Some people get turned off by the concept of having to roleplay a "faith"...but what is faith other than a personal set of beliefs that you use to deal with your daily life? It is encouraged that the members of Pantheon find and follow a deity (or deities) of their choice mainly because it is an excellent tool to give players a set of motivations. In other words, choose to follow a God of War, and you have instantly gained a set of suggestions on how to act, it makes roleplaying that much easier.
However, even those who wish to roleplay atheistic or agnostic are welcome to join, for everyone is entitled to being included in a roleplaying friendly environment. Yes the guild's main goal to to bring back the word of the Old Gods to all of Hibernia, but there are always skeptics, even in the very groups that deliver the message. Just as with good and evil, a balance of extremes is necessary for creating dynamic relations, and those who do not believe in gods or religion satisfy this role quite nicely.
In the end, everyone needs to learn for themselves, and sometimes by watching others demonstrate how they do it (I certainly remember my first time sitting down at the table with a group of strangers for tabletop roleplaying...I hardly said a word) they can decide for themselves just how they want to play and act everything out...and how commited they think they can be. Even the most hardcore roleplayers like a break from being in-character now and again, don't think that you're not cut out to roleplay just because you get sick of it after a short time. Roleplaying for a short time is better than not at all.
Lastly, remember that you chose a Roleplaying Server, and the others on there...even if you don't think they roleplay...deserve to have their wishes respected. It is your choice whether you roleplay or not, but it is not really a choice for others to listen to you. You may not roleplay, but the others around you chose this server for one reason or another, and probably not the least of which was to be in a Roleplaying conducive environment. If you don't have anything nice (roleplaying) to say...then try not to say anything at all, it is far more respectful to those who wish not to hear blatantly non-roleplaying speech. That is one thing most people forget on this server; and although you are entitled to those days where you just want to relax out of character, you need to be mindful that the others around you might not.