William Louis Petersen is an American actor and producer, best known for playing Dr. Gilbert "Gil" Grissom on the hit CBS series .
He has also portrayed President John F. Kennedy in the 1998 TV film The Rat Pack. Petersen is notoriously selective about the film roles he chooses, and has turned down roles in several films that went on to become modern classics.
Petersen, the youngest of six children, was born in Evanston, Illinois, to parents who worked in the furniture business. His father was Danish-American Arthur Edward Petersen, Sr. and his mother was German-American June Hoene Petersen. He has two brothers, Arthur, Jr. and Robert, plus three sisters: Anne, Mary Kay, and Elizabeth. He graduated from Bishop Kelly High School in Boise, Idaho, in 1972. He was accepted to Idaho State University on a football scholarship. While at Idaho State, Petersen took an acting course which changed the direction of his life. He left school along with his wife, Joanne, in 1974 and followed a drama professor to the Basque country where he studied as a Shakespearean actor. Petersen was interested in Basque culture and he studied the Basque language, Euskera, and gave his daughter the Basque name Maite Nerea ; she was born in Mondragón 1975. Petersen returned to Idaho intent on being an actor. Not wanting to work a non-acting job in Idaho, he returned to the Chicago area, living with relatives. He became active in the theater and earned his Actors' Equity card. He performed with the Steppenwolf Theatre Company, of which he has been an ensemble member since 2008, and was a co-founder of the Remains Theater Ensemble which also included other prominent Chicago actors Gary Cole and Ted Levine.
He is usually credited without his middle initial .