The impact of presidential debates on candidate evaluations remains an open topic. Research has long sought to identify the factors that matter most in citizens' responses to debate content, including what candidates say, how they say it, and the manner in which they appear. This study uses detailed codings of the first and third 2012 presidential debates to evaluate the impact of candidates' verbal and nonverbal behaviors on viewers' second screen responsetheir use of computers, tablets, and mobile devices to express their reactions to the viewing experience. To examine the relationship between candidates' on-screen behaviors and the social media response, we conduct generalized least squares regression (Prais-Winstein estimation) relating two data sources: (a) a shot-by-shot content analysis coded for rhetorical/functional, tonal, and visual elements of both candidates' behavior during the debates, and (b) corresponding real-time measures of the volume and valence of online expression about the candidates on Twitter. We find that the nonverbal communication behaviors of candidatestheir facial expressions, physical gestures, and blink rateare consistent, robust, and significant predictors of the volume and valence of public expression during debates, rivaling the power of memes generated by candidates and contributing more than rhetorical strategies and speech tone.