Jeanne de Saint-Rémy de Valois was a trashy con artist that was romanticised by this movie. She was petite, pretty, brunette, blue eyed, greedy and very manipulative! Other than the blue eyes, I can't think of a better actress to suit the role of Jeanne de Saint-Rémy de Valois.
The writers really made Jeanne de Saint-Rémy de Valois into some sort of sympathetic hero that deserved the audience’s empathy and concern. The real Jeanne de Saint-Rémy de Valois could not have been further from such a person; she was well, excuse the expression, scum.
What, just for once in your life can't you be serious?
IMO Hillary Swank is the weakest link of the movie. She speaks with a thick yank accent unlike the rest of the cast who are british bar Adrien Brody and Christopher Walken. Anyway Adrien Brody's character is just a supporting role, besides Adrien does better with the accent. Christopher Walken's accent is fine, being Count Cagliostro a foreigner, then makes sense the fact that he speaks with a different accent, he could have tried an italian accent, though. Besides Hillary Swank doesn't fit this role. I mean, she's fine playing other kind of characters but she's not believable at all as a countess, a member of the aristocracy. She can't play convincingly a decadent and snobbish aristocrat unlike Simon Baker, Brian Cox or Joely Richardson.
You are right, Simon Baker is an aussie. Anyway for one reason or another aussies, unlike yanks, doesn't seem to have problems to replicate convincingly the british accent. There are lots of examples of that. From Russell Crowe to Hugh Jackman. Simon Baker is another one.
What I didn't get is: why hire an actress with an English accent so many others were attempting in the film, only to Americanize her accent? I guess so she wouldn't stand out among Americans trying to sound so "period piece" in their speech. ;)