Ryan Phillipe really made a transformation for this role
It's kind of hard to believe that the same guy who played the skinny pipsqueak in Cruel Intentions played Parker in this film just one year later. It was a real transformation, from young metrosexual to grizzled criminal.
I actually heard a story that he was dead-set on playing the role and kept pestering Chris McQuarrie to cast him. McQuarrie thought he was all wrong for it, however, and told Ryan to "go be in a movie people are actually going to see."
Ryan wouldn't take no for an answer though and put on 25 lbs and grew a beard and McQuarrie, impressed by his dedication, went ahead and put him in the movie.