BACKGROUND: Behaviour change interventions delivered in real-world settings could be vulnerable to threats to internal and external validity if methodological considerations are overlooked. The primary dental care setting is a difficult environment within which to deliver research protocols presenting unique challenges for robust scientific research delivery. Intervention fidelity strategies are an important mechanism to improve the scientific rigor of such studies. Feasibility studies provide a vital opportunity to refine and optimise research processes and implementation strategies before embarking on larger scale projects. This paper sets out the development of a comprehensive intervention fidelity strategy guided by The National Institutes of Health Behavior Change Consortium.(BCC) recommendations. METHOD: Using observations (53 h) and qualitative interviews (17 patient interviews and 2 staff interviews) conducted during the delivery of the RETURN feasibility study (InteRventions to rEduce inequaliTies in the Uptake of Routine deNtal care), an intervention fidelity strategy was developed for use in the RETURN main trial. RESULTS: A comprehensive intervention fidelity strategy was developed, structured around the five domains of the BCC's recommendations (design, training, delivery, receipt, enactment) and attending to the goals pertaining to each of those domains. This paper sets out the fidelity strategy implemented in the RETURN main trial. IMPLICATIONS AND CONCLUSIONS: The RETURN fidelity strategy was influenced by the unique research environment the primary dental care setting presents. The strategy could serve as a blueprint to other researchers conducting research in similar settings. It is also intended that this strategy is read alongside the RETURN results upon their publication. TRIAL REGISTRATION: ISRCTN10853330, registered: 07/10/2019.