Here's what the actress told E! Online:
"Her character's kind of delicious and sort of Catherine-esque in her manipulations," she previews. "Where Catherine uses her children, she still ultimately loves them and wants their happiness. With Marie de Guise, she does use Mary, and it's questionable whether or not she's emotionally invested in Mary's emotional well-being."
Here's an extended promo for an all new Reign "For King and Country" (airing on Dec. 19):