If i were ever to write a proper (by my standarts :D) character, most of the flesh wold go to their backstory. A well-written background in cojunction with a fleshed-out personality and several key goals and beliefs (what do they want from life, what do they count wrong and immoral or praise-worthy and virtuous) may be just enough for about any situation that may occur during the course of an RP. 

Though im well known for being an idiot airhead, so this might just be a heap of nonsense or painfuly obvious information.