There is a growing volume of research on task-based language use; however, the nature of "task proficiency" has not yet been clearly defined. In order to gain new insights, this study examines the relationship between the process of communication in an L2 and a task outcome by analysing lexical density, as obtained from the pattern of a concordance of L1 "private speech" (i.e., self-directed speech). Lexical density is used to analyse the nature of interactive task performance. L1 private speech is a means of observing the way interlocutors make decisions on a moment-by-moment basis. Data were produced by pairs of Japanese college students during a map-completion task in English. The results clearly indicate that the pairs who achieved the best task results were, regardless of their English proficiency level, good at grasping the status of their task performance in both problematic and non-problematic situations. This requires reconceptualisation of L2 proficiency as an emergent property of dialogic processes.