Widespread incorporation of English Medium Instruction (EMI) in higher education has created the need for teacher development initiatives (EMITD) to prepare university lecturers whose first language is not English to teach their content subjects in English. The planning and implementation of such initiatives has frequently been assigned to university language services and the language specialists that collaborate with them. Existing research provides information regarding planned EMITD initiatives, yet there is little research exploring how planned programs are interpreted and implemented in interaction. Such information is vital for quality assessment and for ongoing development. This conversation analysis-informed case study contributes direct evidence regarding how a planned EMITD process is implemented in interaction. Video-recorded interactional data from an EMITD process at a Catalan university are analysed. Participation and notions of interdisciplinary collaboration frame analysis of negotiation sequences. The findings reflect significant modification of the originally planned collaborative process and the roles of participants and the co-construction of participation frameworks that do not support interdisciplinary collaboration. These findings suggest the importance of examining EMI training processes on a local level to better inform future EMITD initiatives and the need for development of training initiatives for EMI teacher trainer-mentors on the part of university language services.