The Official Site has an interview with prequel casting director Robin Gurland. Here's an interesting excerpt:

Portman and Christensen, as well as the other principals, had a wealth of backstory when developing their characters. For other, smaller roles, the characters are much more vague, offering room for interpretation from the supporting actors.

"When George Lucas writes a script, you end up with a series of vignettes that stand alone, or that are very compact scenes," explains Gurland. "Oftentimes, an actor will come to it without a lot of backstory. The character appears, has a function within that vignette, but doesn't exceed that. It's up to the actor to create and fill in the blank spaces of that character. That happened with Alethea McGrath, for example, as Madame Jocasta Nu. She created this huge backstory on Jocasta -- where she was born, how she got there, what her education is -- and it enriches the part, because it gives her more to work with when she's doing her scene."

With characters that were already established yet required new actors -- like the teenage Owen and Beru -- the previous films of the saga provide the needed background, though the actors still often bring in their own histories. "Joel Edgerton did that with Owen, and he's a Star Wars fan. He knew about the original Uncle Owen, and how he interplayed with the rest of the characters, and the little nuances that Phil Brown had created for the original performance."

Edgerton wasn't the only self-proclaimed Star Wars fan in the cast, something that Gurland recalls with a smile. "These guys come to the table knowing so much about Star Wars. Half the time, Jay [Laga'aia], Joel, Ewan or Hayden will start talking about Star Wars, and it's just so lost on me," she laughs.

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