Right now I'm working with a character who we first meet when an important mission goes south. Then, he is simply a threat, and we know he must be a particularly dangerous threat because of who he is working with. His primary function is to amp up the tension as we realize that things have gone out of control.
Later, through various twists of fate, we're working with each other. While planning a mission, there is a beat that doesn't drive the plot forward, but dwells on the characters' pasts.