People increasingly turn to social media to augment their broadcast viewing experience with a parallel stream of information and opinion. Known as "social watching," the practice of integrating broadcast media and social media has become routine for many citizens tracking live events and breaking news. In a controlled laboratory study, we examined how interactivity and exposure to social media opinions influence a sense of community, attitudes and discussion elaboration. The results suggest that receiving positive feedback to social media posts instills a psychological sense of community in the poster, and this feeling of connectedness is related to greater elaboration of the civic social media discussion. Secondly, the study found support for conformity effects. The third contribution of this work is a better understanding of how the valence of others' social media posts and the user's posting activity influences cognitive elaboration of social media discussions during social watching in civic contexts.