Jackie Chan complained politely about the sword skills of John Cusack: "(He) needed a little more work."
Both Lorie Pester's and Sharni Vinson's voices are dubbed over, Vinson's by Kate Hawkins and Pester's by an unnamed actress. While Pester is French and has a slight accent in English, Vinson is Australian, thus a native English speaker, and has put on an American accent (used by the Romans in this film) for roles numerous times. Thus it is likely the voices were dubbed not out of concerns regarding the actresses' accents, but out of the convenience of using a voice actress based in China rather than flying the two back to record ADR. Pester does provide her own voice in the French dub of the film.
Was featured on episode 207 of the comedy podcast How Did This Get Made with special guests Daniel Radcliffe & Erin Darke