OBJECTIVE: To understand participants' experiences and understandings of the interventions in the trial of a computerised decision support tool in patients with atrial fibrillation being considered for anti-coagulation treatment. DESIGN: Qualitative process evaluation carried out alongside the trial: non-participant observation and semistructured interviews. PARTICIPANTS: 30 participants aged >60 years taking part in the trial of a computerised decision support tool. RESULTS: Qualitative evidence provided the rationale to undertake a decision to discontinue one arm of the trial on the basis that the intervention in that arm, a standard gamble values elicitation exercise was causing confusion and was unlikely to produce valid data on participant values. CONCLUSIONS: Qualitative methods used alongside a trial allow an understanding of the process and progress of a trial, and provide evidence to intervene in the trial if necessary, including evidence for the rationale to discontinue an intervention arm of the trial.