This article examines the practice of "co-participant completion" in Japanese conversation, and explores what kinds of resources are mobilized to provide the opportunity to complete another participant's utterance-in-progress. It suggests the following observations as potential characteristics of Japanese co-participant completion: (i) Syntactically-defined two-part formats (e.g. [If X] + [then Y]) may not play as prominent a role as in English; (ii) The majority of cases of co-participant completion take the form of 'terminal item completion;' (iii) Locally emergent structures like 'contrast' and 'list' as well as 'unprojected' features of turn construction often play an important role in enhancing the opportunity for completing another participant's utterance-in-progress. The article then discusses the implications of these findings for the investigation of the mutual bearing of grammar and social interaction. In particular, the discussion focuses on what we can learn from the practice of co-participant completion about how projection of turn-shapes is accomplished in Japanese conversation.