Using a case study approach, this article examines the challenges faced by a cooperating teacher and student teacher during their collaborative planning conversations. I draw on several analytical frameworks, including sociolinguistics, speech act theory and discourse analysis, to analyze the cooperating and student teacher's efforts and struggles to negotiate shared instructional plans. Analysis of several videotaped planning conversations shows how the two teachers' conventional interpretations of their roles as expert and novice planners limited their ability to discuss different approaches to teaching English. The discussion underscores the need to improve the mentoring relationship between cooperating and student teachers by better preparing them for their expert-novice roles. I conclude that cooperating and student teachers could engage in more educative planning conversations if they broadened their understanding of their roles as expert and novice planners and of the definition of expertise itself. (Contains 3 notes.)