The "Mad Men" actress was originally cast in a supporting role as a stripper Krissi Cates, but now she'll take a lead role as Patty Day, the murdered mother of Charlize Theron's character Libby Davis.
Samantha Morton dropped out leaving the role open, and producers offered the part to Emmy-nominated Hendricks.
As in the book, the film follows Libby as she recounts the massacre of her family at the hands of her brother Ben. Shown through a series of flashbacks, Libby discovers she may not have seen the event as it really was.
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