William Louis Petersen (born February 21, 1953) is an American actor and producer, best known for playing Dr. Gilbert "Gil" Grissom on the hit CBS series CSI: Crime Scene Investigation. He portrayed President John F. Kennedy in the 1998 TV film The Rat Pack.
Petersen was born in Evanston, Illinois, the youngest of six children of June (née Hoene; 1909-2006) and Arthur Edward Petersen, Sr. (1907-2004), who worked in the furniture business. He is of Danish and German descent. 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 (meaning My Beloved); she was born in Arrasate - 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.
In 1985, Petersen received his first break when he starred as a Secret Service Agent gone rogue to avenge his mentor in William Friedkin's 1985 action film To Live and Die in L.A. The following year, Petersen starred in the first Hannibal Lecter film, Manhunter, playing FBI agent Will Graham. Because his role in Michael Mann's Manhunter was so emotionally exhausting, he did everything he could to rid himself of Graham after finishing principal photography. He shaved off his beard, cut his hair and dyed it blonde. He also claims to have done this because, while rehearsing for a play in Chicago, his dialogue was always coming out like Graham's; he dyed his hair so he could look in the mirror and see a different person.