In 'expert-to-non-expert' interactions, one of the distinguishing features is that there is no or very little shared knowledge between the subjects. This situation may become particularly challenging when the unshared knowledge is of a very technical kind, as the likeliness of misunderstandings or unsuccessful communication becomes very high. This is particularly true of interactions between patients and physicians. In the course of such interactions, physicians are expected to inform, advise and persuade patients regarding their health problems. It is especially when differences of opinion emerge that physicians need to be persuasive, but it is also then that this may become very difficult, as the patient does not share the medical expertise of the physician. At these moments, one of the most powerful means of persuasion in the hands of physicians is their professional ethos, or authority. The paper presents partial results of an ongoing research project aimed at describing the ways in which physicians construct their professional ethos in interactions with their patients, and how they use it to reconcile patients' diverging opinions with their own. The analysis is carried out on a corpus of video recordings of doctor-patient interactions and it is aimed at identifying different persuasive strategies based on the professional ethos.