On paper, the interview went great and I had no problems with the whiteboard questions or any of the other questions. However, his body language was clearly not vibing with mine. He may have felt I came off too aggressive and overconfident. For the record, I did not think it was confrontational, in fact the conversation itself was pretty easy going and fun as far as technical interviews go.
I've been thinking about it and trying to come up with a way of calibrating my body language in the moment to make the other person more at ease. If anyone has any suggestions on reading, I'd love to hear them. I've read that FBI agent's book  since then and found it a little bit helpful, but nothing earthshattering. The author is more concerned with cracking/breaking people than making them comfortable.
Incorrect. The author is retired FBI Agent, Joe Navarro,  an expert on reading and interpreting non-verbal body language. One of the cornerstone of non-verbals, explained in WEBIS, is the comfort/discomfort paradigm. ,
Is a person comfortable? Why not? Is there a mismatch between verbal communication and non-verbal actions? This is where rapport comes in, the ability to make someone feel at ease and something taught by the FBI when interviewing. They don't "break" people, they charm them by putting them at ease, let them drop their guard and inquire further. The earth shattering point of WEBIS is everybody exhibits these body language responses and I consider it to be a sort of human response API regardless of country, age or sex.
As for eye contact. Hold too little contact and you appear submissive. Too much and you may be signalling to another person you are aggressive.
 backgrounder by Octavio Blanco "This former FBI spy hunter fled Cuba at age 9" ~ http://money.cnn.com/2015/10/27/news/economy/joe-navarro-cub...
 Navarro, 2007.
 read Charles Fifield, "Setting the Sale Through Persuasive Communication"
I confess I must have misread his points because many of the stories are about criminal interrogations (although there are plenty of business stories too). I just bought his other book Louder Than Words and I will be reading that looking for the charm/rapport aspect instead of "breaking".
No idea how legal this is though, download at your own risk.