Jim Caviezel is Jack
Jim Caviezel (The count of Monte Cristo, Passion of the Christ, Frequecy) not only looks the part but also has the acting chops to bring Jack to life.
I can just see Jim Caviezel walking down the street Happy in "All The Rage" after being dosed and becomming the dark Jack as the drug kicks in. Pity on the fools who get in his way and how good he could fight the drug when he see's his girls. Then his dark side come out when he understands what has been done to him and how he proceeds.