For his big screen debut in "Alpha Dog," Justin Timberlake had to bulk up considerably -- damn he looks good!

To play Frankie Cassavetes, a physical trainer in the real world, the slim Justin had to get to the gym and start flexing his muscles -- beginning with just 5 lb. weights. The hardest part -- building his core before starting to lift real weights. "You first look at it like, it's five pounds, it's a girl's weight," Timberlake told Moviefone, "and then all of a sudden, you feel like a girl."

Cameron Diaz is definitely missing out now!