Previous research illustrates the collaborative nature of adolescents' multimodal composing processes. However, few studies have specifically focused on how different modes influence student interactions over time. This study examines how multiple modes (e.g. text, music, visuals, and animations) mediated middle schoolers' composing processes as they worked in small groups to create multimodal science fictions. Situated in an afterschool program, each student selected the role of writer, scientist, or designer. Data sources included screen capture video, semi-structured interviews, and multimodal products. Qualitative data analysis involved the constant comparative method to establish codes for types of interactions and the mediating modes as a case study small group collaboratively composed. Findings indicate: (1) students were inclined to provide short responses to move on with composing practices; (2) group discussions while multimodal composing followed three stages: mode and story exploration, mode-story integration, and mode-story completion; (3) multimodal comics fostered the most discussion; (4) different modes supported self-oriented and group-oriented contributions in unique ways. This study contributes an initial understanding into how different modalities mediate students' interactions and offers implications for scaffolding peer interactions during multimodal composing processes.