For me, the easiest way to do it was to let my character (Excelsior) flow as an extensive of my personality. I decided what I wanted him to look like ahead of time, then I looked for photos that fit my idea of him on the internet and had a siggy made up. This helps me visualize him and make him more real to me, also. But as far as the RP goes, I RP him much as I would myself. He's a very polite, well spoken person who doesn't want to offend but, well, you know he's probably just looking out for himself in the end. It's fun to interact with the other characters, just let the conversation flow and imagine yourself as reallly being a student at Hogwarts and it comes easy.