OK, first the Peterson business. He'll be leaving after the tenth episode of the upcoming season, though Tassler says "this won't be the last we see of him." Why did he leave? Because he's an artist! "Billy is an extraordinary guy. Let's not forget his roots and origin in theater; he's still an artist and very passionate and committed to the show [as an executive producer]. (His departure) is just an artist's choice to change his life." He'll be working on a play soon, she said.
The character that will replace him -- he's still being cast, though in the scrum, she said that they approached John Malkovich's people about the role, but he turned them down, and that Laurence Fishburne would be "great for the role," and was a name that they considered -- is an outsider to the Vegas CSI unit, and won't be the leader of the team at the outset. Through testing, he has found out that he has the same genetic profile that has been found in serial killers, and he's making "a journey to discover his true character." There will be a two-episode overlap between the new character's arrival and Peterson's departure, according to Tassler.
