  Live streaming platforms and existing viewer participation tools enable users
to interact and engage with an online community, but the anonymity and scale of
chat usually result in the spread of negative comments. However, only a few
existing moderation tools investigate the influence of proactive moderation on
viewers' engagement and prosocial behavior. To address this, we developed
StoryChat, a narrative-based viewer participation tool that utilizes a dynamic
graphical plot to reflect chatroom negativity. We crafted the narrative through
a viewer-centered (N=65) iterative design process and evaluated the tool with
48 experienced viewers in a deployment study. We discovered that StoryChat
encouraged viewers to contribute prosocial comments, increased viewer
engagement, and fostered viewers' sense of community. Viewers reported a closer
connection between streamers and other viewers because of the narrative design,
suggesting that narrative-based viewer engagement tools have the potential to
encourage community engagement and prosocial behaviors.
