The pair will play a father and son in the indie thriller, reported The Wrap.
Hirsch, 29, and 68-year-old Cox star as coroners who receive the body of an unidentified woman.
The corpse shows no cause of death and as the pair investigate the mystery, chilling secrets are revealed.
Andre Ovredal (Troll Hunter) is to direct the film.
Martin Sheen was previously rumoured to be starring in the movie.
Hirsch was recently charged with felony assault on a female film executive at the Sundance Film Festival.