The process of creating stories with others is highly engaging but labor-intensive, especially because satisfying narratives often require a central organizer to provide structure and scaffolds. To make this task easier for the organizer and the participants, we propose the use of context-awareness as a way to scaffold participant contributions in a collaborative storytelling experience. We have created Cast, a platform that allows an organizer to concisely define a story script that is opportunistically deployed to users through a context-aware mobile app. Leveraging the contexts of users to define their roles within a collaborative story provides them with scaffolds that promote the development of a compelling narrative, and automating this process prevents burdening the organizer. A pilot study demonstrated that stories developed with Cast are engaging yet low-effort for participants, showcasing the potential for context-awareness to make collaborative storytelling more accessible without sacrificing enjoyability.