"Deadwood" alum Larry Cedar has also come on board the NBC TV series, according to The Hollywood Reporter.
The eight-episode first installment of the anthology series is described as a "gripping in-depth dramatisation" of the famed murder case on brothers Lyle and Erik Menendez that took America by storm in the early 1990s.
Graham will play Judalon Smyth, an emotionally fragile woman who is having an affair with the Menendez brothers' psychologist, Dr Oziel.
Reaser will portray Deputy District Attorney Pam Bozanich, who is assigned to the murders of Jose and Kitty Menendez, while Cedar is set as Milton Anderson, Kitty's older brother.