Today, learners’ interaction and collaborative task performance have attracted increasing attention from language teachers and researchers. The present study investigated whether collaborative pre-planning, task complexity manipulation, and language proficiency level play a role in learners’ interactions. To this end, 128 EFL learners from two different language proficiency levels carried out three different tasks, whose complexity was manipulated based on Robinson’s task complexity framework. Retrospective semi-structured interviews were conducted which led the researchers to a better understanding of the unobservable underlying processes they underwent in the pre-task planning stage. The learners’ interactions were closely examined, analyzing all their language related episodes quantitatively as well as qualitatively. The results partially supported the Cognition Hypothesis and highlighted that cognitively demanding tasks provide more learning opportunities as learners confront more challenges, compared to undemanding tasks. The results were highly revealing about the process learners undergo in the pre-task planning stage, which can be considered by applied linguists, language teachers and material designers in providing considerable learning opportunities.
