In Christopher Nolan's The Dark Knight, the character of Rachel Dawes is played by two different actresses. Maggie Gyllenhaal replaces Katie Holmes in the role, something that was necessary due to a scheduling conflict. However, this change also allowed for a shift in the character's arc.
In the comics, Rachel Dawes is Batman's love interest. She is an attorney who works tirelessly to help Gotham City's underprivileged. She shares Batman's moral compass and sense of justice. However, she disapproves of his methods as Batman and their relationship is often strained because of it.
Nolan's version of Rachel Dawes is more ambiguous. She still works as an attorney, but her focus is on prosecuting members of organized crime. This puts her at odds with Bruce Wayne/Batman (Christian Bale), who views himself as above the law. Their relationship is much more complex than in the comics and Gyllenhaal brings a new depth to the character.
The decision to change actresses midway through filming was a bold one, but it paid off in terms of story development. Rachel Dawes is an important part of The Dark Knight and her arc helps to drive the film forward.