In this paper we offer a proposal to enable agents to discuss the suitability of plans based on an argumentation scheme and associated critical questions. The detail encompassed by the argumentation scheme means that there is a large number of critical questions, and so dialogues may in principle be very lengthy. To improve the efficiency of dialogues we present two strategies for selecting questions. We have implemented the system and here present results showing how both strategies are effective in reducing the number of questions required to reach agreement, although their relative effectiveness is dependent on characteristics of the problem.