I envision Derpy as a shatterbrained girly-girl, with no attention span, but a big heart. Perhaps a bit immature. Just a really fun mare to be around.
Lyra I see as a somewhat chaotic pony; an energic tomboy, with no care for others' opinions, but completely in the power of her emotions. Bon Bon I see as her opposite, caring a lot about her own appearance and generally a bit of a valley girl, perhaps with some jerk-tendencies, but genuinely nice if one gets to know her.
With Octavia, I completely go with the idea that she's Pinkie Pie's sister, so I think she's trying her best to become a stoic upper-class mare and leave her time on the rock farm behind. I still believe she loves her family very much, she might just think they're in the way of her career.
Then there's the almost completely unknown background pony, known as Lotus Blossom (or Frou Frou, depending on who you ask). She's the mare who got her mane ruined by the CMC in "The Show Stoppers" and ran away screaming... in a squeaky voice. I fell in love with her at once and immediately found a personality for her: due to her unusual voice, Lotus has a hard time being taken seriously, so one day when she was a filly, she simply stopped trying. From then on, she just did whatever she wanted, whenever she wanted and used her wits and unique charm to get what she wanted. She makes a living of singing the strangest songs, but for the most part just travels around Equestria, having a blast. For added cool, lotus flowers actually symbolize getting through a rough patch in your life and moving on. Her cutie mark, being one of these, symbolize her ability to make the best of any situation.
Haven't really given the other known BG's much thought, but I love all the different fan-interpretions.