The current K-12 educational system often does not engage students in interdisciplinary learning. To address this need, we developed an integrated STEM + L (Science, Technology, Engineering, Math, and Digital Literacies) afterschool programme in which middle school students took discipline-specific roles (writer, scientist, artist, and engineer) and worked in small groups to produce multimodal science fictions. The study aims to investigate the effect of discipline-specific roles on students' interdisciplinary and collaborative learning processes. Two groups were chosen as comparison cases because their final products indicated low and high levels in integrating science and literacy respectively. Analysing and visualising students' individual role-changing patterns and examining their discourse in role-specific interactions revealed the following features for high-quality interdisciplinary learning: (1) Students need to be willing and able to enact not only expected roles, but also other roles; (2) Students need to develop awareness of their own and others' role enactment; (3) Collaboration between students with different roles may follow a possible trajectory from solo-construction, to co-construction, to re-construction. This study also proposes instructional strategies related to the design of discipline-specific roles to facilitate interdisciplinary learning in STEM education.