Objectives: While general practice involves supporting patients to modify their behaviour, General Practitioners (GPs) vary in their approach to behaviour change during consultations. We aimed to identify mechanisms supporting GPs to undertake successful behaviour change in consultations for people with T2DM by exploring (a) the role of GPs in behaviour change, (b) what happens in GP consultations that supports or impedes behaviour change and (c) how context moderates the behaviour change consultation. Methods: Semi-structured interviews with academic clinicians (n = 13), GPs (n = 7) and patients with T2DM (n = 16) across Australia. Data were analysed thematically using a realist evaluation approach. Results: Perspectives about the role of GPs were highly variable, ranging from the provision of test results and information to a relational approach towards shared goals. A GP–patient relationship that includes collaboration, continuity and patient-driven care may contribute to a sense of successful change. Different patient and GP characteristics were perceived to moderate the effectiveness and experience of behaviour change consultations. Discussion: When patient factors are recognised in consultations, a relational approach becomes possible and priorities around behaviour change, that might be missed in a transactional approach, can be identified. Therefore, GP skills for engaging patients are linked to a person-centred approach.