Interactivity with users represents a key feature of transmedia storytelling. This research revolves around the way viewer engagement influences the content of an interactive Web series-namely, Si fueras tu. The series used a strategy based on social networks to encourage viewer participation in the decisions of the protagonist. Research uses a mixed study methodology that includes (1) an analysis of turning points, (2) an analysis of characters, (3) content analysis based on viewers' comments on social media, and (4) indepth interviews with project managers to illuminate the process of building the transmedia strategy. Findings confirm that audience participation influences the content of the fiction series, albeit in a limited way, with the main aim being to retain the audience as part of the marketing strategy.