Citing her emotional instability and tendency to go off-script, Quinn exiles Carried from the interrogation and kicked things off himself. This answers our questions about whether her move against Brody was perceived in the agency as a good idea.
Quinn appears to be progressing competently through the interview, but even after he lays his cards on the table by showing Brody the incriminating video, his subject doesn't yield an inch. In apparent frustration, Quinn whips a pen knife out of his pocket and stabs Brody through the hand. Saul, Carrie, and someone with a syringe full of [tentanus? morphine? truth serum?] rush in to remove Quinn.
It seems we were treated to some political commentary about the perils of the rogue interrogator, the poorly supervised guard, the fragility of the American legal process (this can apparently all be explained through Quinn's off-hand comment about broad national security authority from Congress). Quinn, rather than being banished from the CIA and charged with assault, calmly returns to the observation room and takes a seat at Saul's side.
As Carrie assumes the reins of the interrogation and begins her turn in the room by offering Brody a fresh bottle of water, Quinn's "every good cop needs a bad cop" line implies that he knew exactly what he was doing. Carrie, too, is ready to play along; rather than walking off the case after such a display of brutality, she performs her role with an expertise that shows she's been there before.
In a skillfully executed maneuver, Carrie applies her knowledge of Brody's relationship with Abu Nazir to emulate what she knows of Brody's mentor. Like Nazir before her, she "saves" him from torture, offers kindness and empathy, and then makes the big sell to Brody's conscience to convince him to cooperate with his new captors. She also uses their romantic history to "turn" her prisoner.
Touting the psychic virtues of coming clean, Carrie offers a few pearls of apparent truth, telling Brody she wants him to leave his family and be with her. And despite the fact that she just had him arrested, we still believed her.