Abstract Background Recruitment to surgical randomised controlled trials (RCTs) can be challenging. The Sunflower study is a large-scale multi-centre RCT that seeks to establish the clinical and cost effectiveness of pre-operative imaging versus expectant management in patients with symptomatic gallstones undergoing laparoscopic cholecystectomy at low or moderate risk of common bile duct stones. Trials such as Sunflower, with a large recruitment target, rely on teamworking. Recruitment can be optimised by embedding a QuinteT Recruitment Intervention (QRI). Additionally, engaging surgical trainees can contribute to successful recruitment, and the NIHR Associate Principal Investigator (API) scheme provides a framework to acknowledge their contributions. Methods This was a mixed-methods study that formed a component part of an embedded QRI for the Sunflower RCT. The aim of this study was to understand factors that supported and hindered the participation of surgical trainees in a large-scale RCT and their participation in the API scheme. It comprised semi-structured telephone interviews with consultant surgeons and surgical trainees involved in screening and recruitment of patients, and descriptive analysis of screening and recruitment data. Interviews were analysed thematically to explore the perspectives of—and roles undertaken by—surgical trainees. Results Interviews were undertaken with 34 clinicians (17 consultant surgeons, 17 surgical trainees) from 22 UK hospital trusts. Surgical trainees contributed to patient screening, approaches and randomisation, with a major contribution to the randomisation of patients from acute admissions. They were often encouraged to participate in the study by their centre principal investigator, and career development was a typical motivating factor for their participation in the study. The study was registered with the API scheme, and a majority of the trainees interviewed (n = 14) were participating in the scheme. Conclusion Surgical trainees can contribute substantial activity to a large-scale multi-centre RCT. Benefits of trainee engagement were identified for trainees themselves, for local sites and for the study as a whole. The API scheme provided a formal framework to acknowledge engagement. Ensuring that training and support for trainees are provided by the trial team is key to optimise success for all stakeholders.