Olmos will play a "brilliant, charismatic professor of religious studies,” and Mos Def will portray a "hardened ex-con who claims to have found religion yet seems to continually find himself surrounded by violence."
As previously reported, Hanks is set to appear in all 12 episodes of the season and will play Travis, "a highly intelligent expert on ancient artifacts who gets linked to a series of grisly murders in Miami."
The Showtime series stars Michael C. Hall (Cabaret, Chicago, Skylight) in the title role and in the fifth season, according to Showtime, "Dexter [went] from happily married husband to guilt-ridden single dad." Dexter deals with his loss, a new relationship and his need to kill, which "overpowers his desire for human connection."
Olmos was nominated for a Tony Award in 1979 for his performance in Zoot Suit.
The sixth season is set to debut in the fall. For more information and show times, click here.