The use of psychiatric expertise in criminal trials has generally been construed as problematic despite the fact that there remains little in-depth research in this area. Qualitative research on other forensic experts has indicated the existing literature on psychiatric expert testimony may be simplifying the dynamics that occur in practice between psychiatry and law. This article draws on data generated from 31 qualitative interviews and medico-legal documentation to explore the socio-legal shaping of forensic psychiatrist's expertise during insanity trials in New Zealand. It describes how the hybrid features of the insanity defence shape forensic psychiatrists' practices in this context to the point that their performances as expert witnesses cannot be adequately described as doing 'normal psychiatry'. Rather, adding to the existing body of social studies of forensic expertise, the article exemplifies how the forensic psychiatrists' practices are inextricably linked to the legal context in which they are operating. It concludes by demonstrating that the 'art' of forensic psychiatry involves managing the hybrid nature of the expertise they provide in cases of insanity.