Izzard will appear in eight episodes and Conroy in one installment of the program.
They will act opposite Toni Collette, last year's Emmy Award winner for her portrayal of Tara Gregson, a wife and mother who struggles with Dissociative Identity Disorder.
"Izzard will play Tara's brilliant psychology professor who is at first a DID skeptic, but becomes fascinated with Tara as a subject, leading him to further explore the condition," Showtime said in a news release. "Conroy will star as Max Gregson's mother, a recluse with a compulsive hoarding problem."
John Corbett plays Max, Tara's husband.
Production on Season 3 of the show is to begin in mid-September in Los Angeles for a 2011 premiere.