This paper reports on part of the evaluation of "The Common Learning Programme in the North East", which offered practice-based interprofessional education (IPE) to pre-qualification students. A realistic evaluation approach was used and data collection methods included interviews and observations in an attempt to look into "the black box" of practice-based IPE. The contexts of the sessions covered a number of clinical settings and involved a range of participants. Mechanisms included the content of the sessions and the procedures involved. Findings illustrate the complex and unpredictable ways in which discussions arise and evolve during IPE sessions and how interplay exists both between the contexts and the mechanisms, and between knowledge types. Issues are raised regarding the facilitation of IPE and the influence of the current evidence-based movement on research types. This study highlights the complexity and unpredictability of practice-based IPE and the usefulness of research approaches that look into the black box of educational practice.