A former colleague of Gillian and Cal's is killed on the 6/14/10 episode of Lie to Me. That leads to scenes such as this, as the characters' pasts comes back into play.
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A secret about Gillian's past is uncovered on the episode "Sweet Sixteen." This clips gives us an idea of what that is.
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Lie to Me Season 2 Episode 12 Quotes

Go ahead Gillian Say it. Doyle's back, he killed Andrews and I am next.

Cal Lightman

Jimmy Doyle: I didn't kill Andrews, I only followed him.
Cal Lightman: Well then, the person who did it wanted us to think you did.