At Quantico they'd taught us the so-called classic signals that someone was lying: if they glance up to the right before they speak, or if they won't look you in the eye. None of what I'd learned worked as well as listening to my instincts. I've always been good at ferreting out deception. I'm not entirely sure what my ability to detect a liar is based on -- subtle cues maybe, subconscious awareness, an intuitive talent for reading microexpressions."
-- Marie (narration) (chapter 2)

Importance: From early in her time in the FBI, Marie is incredibly confident in her ability to detect people's motives, as described in this quote. However, as the rest of the novel unfolds, it becomes quite clear that this is in fact her weakness. Despite having several face-to-face conversations with both Ross and Slater, Marie is deeply misled by both of them. Marie's overconfidence ends up being one of her fundamental weaknesses...