I often start with a specific character in mind, but I seldom keep that character as they are when I first join the RP. First, I look at all the information given for the plot/storyline/etc. If any ideas hit I write them down. Next, if there are any characters made or being made I write down traits for a character that would compliment those characters. The traits aren't necessary the kind that would make the characters get along, rather, they would be traits that would prompt interaction between the characters (be it meshing or clashing). After that I look to see what is missing. What does the plot need? How can this character make the story more interesting and help it progress/grow? Once that's done it's usually just a matter of finding some outside inspiration.
Sometimes, when doing the above, I get almost a literal light bulb above my head. I may have already listened to a song, played a game, read a story, watched a show, et cetera which would combine well with the plot/storyline. I try to avoid carbon copies, but certain events/characteristics/traits might be really inspired by a particular source. Regardless, I always make sure a character has some sort of meaning and a means to grow throughout the story so that the audience (or other role-players in this case) can -hopefully- relate to them. I often look to actual people in my life for inspiration for behaviour, speech patterns, or whatever. It's really the best resource for writing a realistic character and you can mix and match certain aspects to get interesting results.
