UK cancer mortality is worse than many other high-income countries, partly due to diagnostic delays in primary care. To understand beliefs and behaviour of GPs, and systems of general practice teams, to inform the Think Cancer! intervention development. An embedded qualitative study guided by behaviour change models (COM-B and TDF) in primary care in Wales, UK. Twenty qualitative, semi-structured telephone interviews with GPs and four face-to-face focus groups with practice teams. Analysis used Framework, results were mapped to multiple, overlapping components of COM-B and TDF. Three themes illustrate (1) complex, multi-level referral considerations facing GPs and practice teams, (2) external influences and constraints, (3) the role of practice systems and culture. Tensions emerged between individual considerations of GPs (Capability, Motivation) and context-dependent external pressures (Opportunity). Detecting cancer was guided not only by external requirements, but also by motivational factors GPs described as part of their cancer diagnostics process. External influences on the diagnosis process often resulted from the primary-secondary care interface and social pressures. GPs adapted their behaviour to deal with this disconnect. Positive practice culture and supportive practice-based systems ameliorated these tensions and complexity. By exploring individual GP behaviours together with practice systems and culture we contribute new understandings on how cancer diagnosis operates in primary care and how delays can be improved. We highlight commonly overlooked dynamics and tensions, experienced by GPs as a tension between individual decision-making (Capability, Motivation) and external considerations such as pressures in secondary care (Opportunity).