She was played by Sonya Salomaa.
Marcia Rideout and her husband, Torrance, were the neighbors of the titular Aurora Teagarden on Honor Street. Throughout the film, Marcia is shown to be sweet natured to Aurora and the other neighbors, as she invited them to a neighborhood barbecue in their honor. As revealed later on, Marcia's sweet demeanor masked her true villainous personality, which came to light in the film's climax.
Two years prior to the film's events, Marcia flirted with married neighbor Brad Reilly, but to her surprise, Brad took her flirtation seriously and pursued her relentlessly. While Torrance was away, Marcia was again pursued by Brad at her home, and it was there that Marcia killed Brad by bashing his head with an aluminum bat. After Torrance found Brad's body, he buried the body in the front yard belonging to Jane Engles, whose house was left to Aurora after Jane passed on. Both Marcia and Torrance kept close watch of the area, and the latter of the couple broke in to the home looking for Brad's skull after it was found by Aurora.
Marcia's reveal came when Torrance subdued Aurora just as she was calling Sally Allison regarding her realization that Marcia had killed Brad. The villainess later appeared to Aurora with her aluminun bat in hand, and confessed to killing Brad, though she stated that she didn't call police because she figured he would stagger his way back up; by the time she realized he was dead, it was too late. After her confession, the evil Marcia handed Torrance the bat and instructed her to kill Aurora in the same fashion that Marcia killed Brad. At that moment, Detective Lynn Smith appeared and held the couple at gunpoint, before handing the gun to Aurora due to going through labor. Marcia and Torrance were both arrested.