Mirror your interviewer's body language. This technique, known as "mirroring," is widely used in the psychological world as a means to gain an interlocutor's trust and make them feel at ease. Good salesmen often use it to increase their chances of closing.
By mirroring an interviewer's movements, tone, gestures, breathing pace and so on, you're basically communicating a message of, "Hey, we're playing the same tune here. We're akin. You can trust me."
Image: FLICKR, TEXAS A&M UNIVERSITY
