The Mental Institution scenes were filmed at the Northampton State Hospital, an actual asylum in Northampton, Massachussets, which was abandoned at the time.
The First six minutes of the film while 'Aidan Quinn' and Annette Bening are at the bedroom doorway discussing her first dream about the missing girl, a black boom microphone can be clearly seen above them following each of their dialogue from behind the door header.
During sentencing, the judge tells Vivian "the State of Massachusetts has declared you insane," thus making him ineligible for the death penalty. In legal parlance, Massachusetts is always referred to as a Commonwealth, and does not practice the death penalty.