Objective: Behavioural weight loss interventions demonstrate success on average, yet participants who respond more slowly may benefit from an augmented, tailored approach. Thus, an augmented intervention for early, slow weight loss responders was implemented. This qualitative analysis explored participants' perceptions of and experiences of the 12-month weight loss programme to inform future intervention development and implementation. Design: Individual interviews were conducted post-treatment. In-person, in-depth interviews were offered prior to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic; videoconferencing interviews were offered following the pandemic onset. Setting: A Midwestern US university workplace. Method: Adults [greater than or equal to] 21 years old with overweight or obesity and prediabetes (n = 39) were recruited and volunteered to participate in a semi-structured interview. Results: Content analysis identified recurring themes among early and early slow weight loss responders. Social support received during weekly group sessions was a critical intervention component. Some early slow responders expressed difficulty with self-regulatory strategies such as setting specific yet attainable goals, and requested additional contact prior to the maintenance study phase -- treatment randomisation at month 5 disrupted group dynamics. Early slow responders who received the intervention via videoconferencing believed the format interfered with their ability to establish friendships with other participants and gain insight and support from others' lived experiences. Conclusion: Weight loss interventions may be more efficacious when considering participant heterogeneity during implementation of behavioural strategies. Findings suggest early slow weight loss responders may benefit from greater emphasis on goal setting and striving, agentic thinking, and social support. Future research is needed to examine the implications of these insights on intervention outcomes.