Rick Santorum is incredulous. Anytime you see a unilateral smile - it's always suspect. An insincere "smile" is asymmetrical. Contempt also has this characteristic. Most otherwise socially adept, educated and experienced people mistake contempt for a coy smile. Not so. Moreover he's not looking at the person who's speaking. We don't look directly at those we don't trust/believe, don't respect or don't like.
What would think if your spouse, business partner, client or child was wearing such a face? I highly doubt Mr. Santorum is making this expression in a premeditated-deliberate fashion. If he were an accomplished actor, I'd still doubt it. Like most of us, he's much less self-aware than he believes. On the very small chance this Body Language is rehearsed and/or deliberate and on cue, I'd advise him to stop immediately.
If you're trying to garner votes, one must look engaged - even if you disagree with what is being said. If you want to take the Michigan Primary (and the Republican Nomination) from Mitt Romney, dial-up your "warm and fuzzy" a bit Rick. The former U.S. Senator of Pennsylvania should be leaning forward in his chair - not backwards - certainly not with his arms crossed. This body language cluster transmits feelings of disengagement, defensiveness, closed mindedness, incredulity and contempt. Rapport will not be developed or fostered this way, yet politicians continue to ask for your votes displaying such nonverbals and salespeople continuously try and close the deal when prospective clients are configured this way - at all the wrong times.