Now if you wanna try out LJ RPing, one thing about it is that you make LJ accounts for certain characters, with the usernames usually based off of them in some way (for example, one of my Lloyd's username is bladesofvalor). And then you'll need icons with different expressions (unless the character is deadpan and has the same expression no matter what the situation), but it's not too hard finding pre-made ones if you're too lazy to make your own/can't find fanart/official art/etc. Unless the series is very obscure, of course :P
As for Original Characters, you can either use original character fanart/icons, or PBs, which is when you use a canon character as a base for your OC (i.e. one of my OCs had Colette as her PB).
Haha this is probably confusing. I've been at it for two years and still haven't gotten the hang of it.