The tone of the new episode would be "dark" - McCallum joked that it saw the young Jedi turn "from a sweet youth into a producer". But he declined to say exactly why Anakin turns into one of cinema's greatest villains.
"Let's put it this way, that's what drives this film."
Episode III will use 60 Australian cast members and up to 300 extras. As well as returning cast members Joel Edgerton and Jay Laga'aia, there are roles for Bruce Spence, Rebecca Jackson Mendoza and Genevieve O'Reilly.
Also back are Hayden Christensen, Ewan McGregor, Natalie Portman, Samuel L. Jackson, Jimmy Smits and Christopher Lee, while Peter Mayhew plays Chewbacca for the first time since Return of the Jedi.
"It's a huge picture and, under today's standards, it's a pretty moderately budgeted film," McCallum said. "But they're a lot of fun to make."
For Episode II, Lucas finished the script only three days before shooting started. This time around, he finished with five days to spare.
"I think they're always tough for him to write," McCallum said. "Especially this one, because it has to tie everything up from the prequels to the original trilogy. And you've got really big themes and issues in this one - how and why Anakin turns into Darth Vader."
The last episode contained two scenes with an Australian flavour - some kangaroo-like animals and a podium shaped like the Opera House. McCallum described them as homages to the country and said more could be expected this time round.
After shooting in 40 countries in 13 years, the producer of the last three Star Wars films said Sydney was the easiest place in the world to make a film, partly because of the strong acting and film-making talent.
"There's no bullshit, no problems, everybody is so enthusiastic and not at all litigious."