Forum discussions have been utilised widely as a means of facilitating learning interaction and social-knowledge construction in online learning. Much research has been conducted on the instructional interventions that benefit asynchronous discussions. Role-playing, or assigning roles to discussants, has been proven effective in promoting interactivity and knowledge construction in the context of both face-to-face and online learning. However, assigning and rotating roles to thousands of learners in massive open online courses (MOOCs) and preparing them to act properly in their roles sounds impractical to MOOC instructors due to work overload. The present study provided three types of role assignment in a MOOC during various course offerings: fall offerings with no role-assignment, spring offerings with partial role-assignment and summer offerings with full role-assignment. Through the examination of the discussion patterns and role-assignment differences among 4,239 students and 5,439 posts in 56 forums, we suggest that partial role-assignment is as effective as full-role assignment. By assigning as few as 10 students with rotating roles, MOOC instructors can leverage this effective strategy while minimising their effort in preparing the discussants and moderating the discussions. These students act behind the scenes and improve the behavioural patterns of asynchronous discussions.