This article examines the interactional work in which two groups of secondary ESL students engaged to achieve and sustain participation in group oral assessment, which is designed to assess a student's interactive communication skills in a school-based assessment context. The in-depth observation of the ways in which participants co-constructed talk-in-interaction led to the discovery of the particular pattern of speech exchange within each group.Within the higher-scoring group, the students engaged constructively and contingently with one another's ideas, demonstrating a range of speech functions such as suggestions, agreement or disagreement, explanations, and challenges, which resulted in opportunities for substantive conversation and genuine communication to be engineered. Within the lower-scoring group, the resulting interactions appeared more structured, apparently as a result of the pre-set prompts that were originally set for the purpose of facilitating within-group discussion. However, a picture emerges of lower-scoring group members naturally engaging in negotiation of meaning over linguistic impasses, which turned out to serve as the stimulus to collaborative dialogue. There is also evidence of lower-scoring group members assisting each other through co-construction both to find the right linguistic forms and to express meaning. The nature of these interactions suggests that the group oral assessment format, as operationalized in this context, can authentically reflect students' interactional skills and their moment-by-moment construction of social and linguistic identity. However, the lack of contingent development of topical talk within the lower-scoring group implies that the assessor's good intentions in providing pre-set prompts may end up restricting students' performance. The risk of such task/topic-related effects on the quality of student discourse and interaction warrants further research. (Contains 3 notes.)