Digital platforms (e.g., Industrial Internet of Things (IIoT) platforms) are on the rise aiming to foster value co-creation in business-to-business (B2B) ecosystems. However, we often observe actors to only hesitantly engage, and activity levels that fall short of expectations. Arriving at a sound understanding of why and how actors decide to engage in co-creation practices is a crucial first step to further promote and facilitate value co-creation in B2B platform ecosystems. This work builds upon the concept of actor engagement, which offers an actor-centric micro lens on the hitherto vague theoretical idea of value co-creation. By pursuing a qualitative approach to theory development based on interviews with platform complementors, we identify factors influencing the formation and extent of actor engagement. Eventually, our research aims to contribute to a refined conception of value co-creation in B2B platform ecosystems by understanding the emergence and nature of actor engagement.