This qualitative study explored non-specific influences on participation in, and outcomes of, a randomised controlled trial. It was nested within a single-blind clinical trial of western acupuncture which compared real acupuncture with two types of placebo control administered to National Health Service (NHS) patients awaiting hip and knee replacement surgery in England. Data collection (2004-2008) was based on narrative-style interviews and participant observation. The results indicate that trial recruitment and retention depend on a set of convictions forged largely as a result of contextual factors peripheral to the intervention, including the friendliness and helpfulness of research centre staff and status of the administering practitioner. These convictions also influence the reporting of the study outcomes, particularly if participants experience uncertainties when choosing an appropriate response. The findings suggest that participants in clinical trials are actively involved in shaping the research process, rather than passive recipients of treatment. Thus the outcomes of trials, notably those involving contact interventions, should be regarded not as matters of fact, but as products of complex environmental, social, interpretive and biological processes. In this paper, we develop and present a 'theory of active research participation' which offers a framework for understanding the impact of non-specific processes in clinical trials.