This chapter reports on a classroom-based quasi-experimental study examining the relationship between interactional moves and collaborative patterns in peer interaction and their effect on L2 development, using proficiency as an independent variable. Participants were from two parallel Grade 10 English classes in Chile, divided into low and high proficiency classes by the school (N = 53). A series of communicative group work activities was designed and implemented during the regular classes. L2 development was investigated by examining past tense usage and vocabulary size. Focus groups from the two classes (n = 10) were used for analyses of interactional features. Results indicated that learners in the low proficiency group (a) provided more corrective feedback and produced more modified output, (b) engaged in more collaborative interaction, and (c) exhibited a greater L2 development gain, than the high proficiency group. In order to interpret the results and conceptualize the links among interaction-collaboration-learning, the findings are discussed in relation to a theory in social psychology referred to as social interdependence theory. It is concluded that while proficiency does have an impact on learners' interactional behaviours, a collaborative mindset - a learner's psychological approach towards the partner and/or task - may be a stronger mediating factor for L2 development.