Circe's introduction should be an adaption of Beauty and the Beasts.
http://www.amazon.com/Wonder-Woman-...bs_sr_3?ie=UTF8&s=books&qid=1218571141&sr=8-3
The gist is that Circe doesn't want Diana to inherit Hecate's soul. Thats why she originally wants to destroy WW. There's more to it which includes the complex history between Hecate and herself but that should be simple to trim down in a movie.
Hecate hates the Greek gods when they alienate her after she helps them kill the old gods. She forms a pact with Circe to grant her more power, as a witch she has magic just not on that level, in exchange for being her champion. This occurs millennia ago.
By the time Diana defeats Ares (Gods & Mortals) and defeats the demons behind Doom's Doorway (Challenge of the Gods) Circe has become the secret master of two Greek islands. One island is populated which she rules for centuries in secret the other she uses as a base. There is an underground movement among the populace who try to rebel against Circe but they are in terrible shape, they dare not tell many people since nobody believes them and Circe will find out quickly who they are with her spies, which could be anyone, and animals she uses as constant surveillance. The animals and her minions kill everybody who gets in their way with ease. They can turn into demonic beasts who have sharp teeth and claws with red eyes that is an evil, distorted version of their true form.
Circe purchases the Greek island in a false identity. No-one goes there as she made sure the public know no visitors are wanted then made sure no-one can talk to her. She never installed phone lines, for instance. People think she's an old crone by now. No-one has any photos or information on her. The populace grow to fear the island and her false identity. They think its cursed. They just don't know the specifics. It's like an urban legend.
Diana goes visits Greece to follow her friends, Vanessa and Julia Kapatelis, who go there to see their relatives who live there. Diana also looks at the ancient sites from Greek culture which has a huge affect emotionally on her since it part of her heritage as an Amazon. The Greek government see her, as well. They see her as their champion.
Circe mainly stays in the shadows plotting for about half the story so when she's revealed its a huge deal. She's much less dramatic in this story then in the present comics. Circe is a brunette, too. The character is much closer to her mythic routes since this is the start of her career as a super-villain in the DCU.