The aim of this qualitative study, nested in a randomized controlled trial, was to assess the mechanisms of the effects in both arms through examining perceptions of the participants about changes in their drinking behavior and their attributions for any perceived changes. We conducted semi-structured interviews with a sub-sample of trial participants. We used thematic analysis to analyze the data. Self-perceived change, mostly positive, was reported regardless of the objectively measured remission status. Participating in the trial itself was a major catalyst for change. Participants in both arms used a variety of similar strategies to make these changes; additionally, for those who received the intervention, both the style of the counselor (for example, the non-judgmental stance) as well as specific elements of the intervention were seen to influence change in drinking behavior. Absence of self-reported change was relatively uncommon and primarily related to the felt need for alcohol (e.g., drinking was perceived to be necessary to maintain good health), or the belief that one did not need to or want to change. Experiences of participating in a trial, the counselor style, and specific elements of a brief psychological treatment, play a role in influencing change in harmful drinking behaviors.