Drawing on recent scholarship integrating usage-based linguistics (UBL) and conversation analysis (CA) in the investigation of second language development, this paper reports on a microanalysis tracing one adult learner's recurring, increasingly frequent, and diverse use of the multiword expression "det er sant" (it/that is true) (DES) in L2 Norwegian interactions over a time span of four months. While the use of DES did not undergo syntactic changes, the analysis revealed that the functions of the expression diversified as DES became more frequent in use. Initially functioning primarily as a means of expressing agreement, DES was increasingly used to preface disagreement and indicate the closing of self-initiated repair sequences. This functional diversification was accompanied by increasing morphophonological reduction indicating an ongoing process of routinization. We argue that these changes, also when non-linear and not complying with the target norm, allowed the learner to participate more actively in managing the flow of talk. The study provides insight into how interaction shapes L2 development and the role of education in providing access to situations that foster increased membership in the speech community.