While educational reforms in introductory physics are becoming more widespread, how these reforms are implemented is less well understood. This paper examines the variation in faculty practices surrounding the implementation of educational reform in introductory physics courses. Through observations of classroom practice, we find that professors' actual practices differ strikingly. We present a framework for describing and capturing instructional choices and resulting variations in enacted practices for faculty who are implementing Peer Instruction. Based on our observations, there are a variety of scientific practices that are supported and modeled in the use of Peer Instruction. In all of the classrooms studied, students were found trying out and applying new physical concepts and discussing physics with their peers. However, there were large discrepancies in students' opportunities to engage in formulating and asking questions, evaluating the correctness and completeness of problem solutions, interacting with physicists, identifying themselves as sources of solutions, explanations, or answers, and communicating scientific ideas in a public arena. Case studies of six professors demonstrate how these variations in classroom practices, in aggregate, create different classroom norms, such as the relative emphasis on student sense-making vs answer-making during Peer Instruction.