Mekhi Thira Phifer ( born December 29, 1974) s an American actor and rapper. He portrayed Dr. Greg Pratt on NBC's long-running medical drama ER and had a co-starring role opposite Eminem in the feature film 8 Miles. He was a regular on the FOX crime show Lie to Me in the role of Ben Reynolds, before season 3, and also starred as CIA agent Rex Mathesons in Torchwood: Mircale Day.