This paper reports on a study that examines how a school teacher, a university researcher, and a professional textbook writer interact when they collaborate in designing materials for English language teaching. Data were collected from interviews with the participants and audio recordings of their interaction in collaborative design supplemented with drafts of the designed materials. The findings showed that the three participants placed different emphases on the design of the teaching materials due to their individual principles, values, and approaches towards English language learning and teaching and materials design. Their values and beliefs were found to inform their practices in collaborative materials design and thus influenced the processes of interaction and negotiation. The study contributes to our understanding of the personalized and diversified principles and beliefs of the stakeholders regarding the development of teaching materials, and suggests that textbook writers and their publishers need to pay more heed to the parties for whom they are producing their materials.