Psychiatric assessments of non-native migrant patients facilitated by an interpreter pose specific communication challenges to all participants. In this study, we developed an original interdisciplinary approach to the verbal and non-verbal practices in this triadic activity. The aim was a data-based description of challenges for clinicians and interpreters, and the identification of relevant strategies. We filmed, transcribed and translated 10 interpreter-mediated consultations focused on the psychiatric assessment of the patient. Subsequently, we submitted the consultations to clinical, interactional sociolinguistic, and interdisciplinary analyses. We identified six challenges for interpreters and clinicians engaged in psychiatric assessments: barely comprehensible and confusing speech, speech about emotions and subjective perceptions, sensitive remarks in relational terms, conclusive clinician interventions, interruptions during interpreter renditions, and non-verbal communication. Attempts by the interpreter to avoid relational offenses (protection of positive face) and to defend the participants' autonomy (protection of negative face) play a major role in these challenges. So does an insufficient awareness of mutual needs by the clinician and the interpreter. We identified specific strategies of inter-professional metacommunication for each challenge. Clinicians and interpreters should be aware of the challenges they may face in triadic psychiatric assessments. They should take a reflexive stance towards their common practices and may consider using metacommunication tools to reach better communicational and clinical outcomes.