Reilly will play Jordan, described as a former D-list actress who is an active partner in the criminal enterprises run by her husband, Frank Semyon.
HBO has also confirmed at long last that Taylor Kitsch and Rachel McAdams are co-starring the sophomore season of "True Detective" opposite Vaughn and Colin Farrell. Production on the eight-episode season has begun in California, HBO said Monday.
Variety first reported that McAdams was in talks for the role in September. Taylor Kitsch also said himself last month that he was joining the cast.
Kitsch plays Paul Woodrugh, a war veteran and motorcycle officer for the California Highway Patrol on the run from a difficult past.
McAdams is Ani Bezzerides, a Ventura County Sheriff's detective whose uncompromising ethics put her at odds with others and the system she serves. Presumably, she goes up against Farrell's Ray Velcoro, who is described as a corrupt detective ruled by Semyon.
The casting on "True Detective's" second installment have dribbled out over the summer and early fall amid feverish speculation about the show's next incarnation. The series is set for a January debut.