I don't actively RP but all of my characters have their own backstories and I have a general idea of their motivations and personalities. The degree of detail varies from character to character. Ironically, my main (a human mage) probably has the least extensive story. He will celebrate his 15th birthday next week and has been around for almost half of my own life; so at this point he's basically just an extension of myself. I'm still unsure about which covenant he will join (none of them really fit in my opinion).
My alts are built on fairly standard archetypes (dwarf hunter and warrior, night elf druid, human paladin and DK, troll shaman) but I still have a general story in mind for each one of them. The most extensive backstory I came up with was for a void elf priest because of the mental gymnastics needed to make the character work.