I had better put this in spoiler code for those who don't want to know. From TV Guide online: Spoiler: Season 4 John Lithgow has signed on to play Dexter's new serial killer. The Emmy-winning actor will play Walter Simmons, aka the "Trinity Killer," so named because he murders in threes. Simmons is a mild-mannered suburbanite who shows up in Miami, on the run from FBI Special Agent Frank Lundy. (Keith Carradine returns as Lundy after a season-long absence.) Dexter Morgan (Michael C. Hall) becomes infatuated with the Trinity Killer's methods, and his ability to evade capture for more than 30 years. Lithgow will appear in all 12 episodes of Season 4, which premieres Sunday, Sept. 27.