 The Dexter prequel gets a new guest star from the Entertainment Tonight Newsroom in Hollywood. I'm Kevin Frazier. Buffy the Vampire Slayer Sarah Michelle Gellar joins the cast of Dexter, Original Sin. Gellar will play Dexter's boss, Miami CSI chief Tanya Martin. The prequel, which follows Dexter during his early days as a forensic analyst who struggles with his homicidal urges, is currently in production and will stream on Paramount Plus with Showtime.