Things aren't always what they appear to be. If you master the principles in "Getting the Truth" and become a "Trained Observer," the good poker player will acquire an edge over many pros.  An excerpt from "Getting the Truth," available at Getting the Truth:

“To become Trained Observers we need to train ourselves to see without filters, to hear without distinguishing, to taste without discriminating, to touch without reacting, and to smell without concluding. Simply put, we need to be objective. Know and identify your prejudices and work hard to set them aside. 

And there should be a sixth sense — one that kind of parallels the other five. One that should always be there, in full use, and operational: empathy. Without empathy, you will not become a Trained Observer. Empathy isn’t feeling sorry for someone — that’s sympathy. Sympathy causes a bias that will keep you from getting the truth. Empathy is much more neutral than sympathy. Empathy is putting yourself in their shoes, using all your senses to experience what they experience. That requires research, patience, work, and commitment. Empathy is a means to understand. It’s only through the peephole of understanding that you can ‘sense’ the true message.”

The poker player who is a trained observer will see that movement, hear that inflection in the opponent's voice, will "sense" when an opponent is confident and when he/she is not. And that will give the trained observer the edge over the competition. Getting the Truth will get you there.