Do you ever wonder why Chris Nolan decided to cast Katie Holmes as Rachel Dawes? Because it's extremely obvious that she couldn't hold her own with actors the caliber of Christian Bale, Michael Caine, and Gary Oldman. I think the reason why Nolan hired Holmes is because Maggie Gyllenhaal was busy and they were behind schedule.
The audition probaby came down to Katie Holmes and Natalie Portman...Chris Nolan probaby chose Holmes over Portman because he didn't want his rendition of Batman to have a connection with the Star Wars prequels because let's face it the Star Wars prequels suck just like the first Fantastic Four film.