Watson was found shot to death April 27, 2018, in a remote area of western Monroe County.
Addie, 54, who was employed as a senior officer at the Moberly Correctional Facility, is being tried on charges of first-degree murder and armed criminal action.
Though married to his wife of 22 years, Melanie Addie, Addie was engaged to marry Watson on April 29 in Columbia. Investigators said that his wife had no idea that he had been dating Watson for at least seven years. The couple met while employed at the prison.
In a short opening statement, Assistant Missouri Attorney General Katharine Dolin, who prosecuting the case against Addie with Monroe County Prosecuting Attorney Talley Smith, said the Watson was living a double life.