Mobile gaming platforms have evolved into participatory digital ecosystems where users simultaneously act as players, creators, and community members. Among these platforms, Roblox represents a distinctive user-generated environment characterized by high interactivity and social collaboration. Drawing on customer engagement theory, this study investigates the effect of interactivity on four dimensions of customer engagement behavior—co-developing, influencing, augmenting, and mobilizing—and examines the moderating role of perceived service quality. Using survey data from 104 Roblox users in Indonesia, the study employs Partial Least Squares Structural Equation Modeling (PLS-SEM) to test the proposed relationships. The findings indicate that interactivity has a significant positive effect on influencing, augmenting, and mobilizing behaviors, but not on co-developing behavior. Furthermore, service quality negatively moderates the relationship between interactivity and all engagement dimensions, suggesting a substitution mechanism whereby high service quality reduces users' reliance on interactivity to remain engaged. This study extends customer engagement research by contextualizing interactivity-based engagement models within user-generated gaming platforms and by revealing a compensatory role of service quality. The findings offer theoretical insights into engagement dynamics in participatory digital environments and provide practical guidance for platform developers seeking to balance interactivity and service quality to sustain user engagement.