William Louis Petersen is an American actor and producer, best known for playing Dr. Gilbert Gil Grissom on the hit CBS series CSI: Crime Scene Investigation, and as Will Graham in the film Manhunter.View newly released photos of William Petersen.William Petersen, the actor with longtime roots on the Chicago stage, is to appear this summer at the Steppenwolf Theatre in a two-person show about a teenager and her reculsive uncle in Costa Rica.