BACKGROUND: Lung cancer has one of the lowest survival outcomes of any cancer because over two-thirds of patients are diagnosed when curative treatment is no longer possible, partly due to later presentation with symptoms to a healthcare provider. OBJECTIVE: To explore the theoretical underpinning of the Scottish CHEST intervention in participants randomized to the intervention group within the CHEST Australia trial. METHODS: A purposive maximum variation sample of participants who received the intervention in the CHEST trial in Perth, Western Australia (N = 13) and Melbourne, Victoria, (N = 7) were interviewed. Patients were asked about their experience of the CHEST consultation, their recall of the main messages, their symptom appraisal and issues relating to help seeking when they develop symptoms. Thematic analysis was conducted to draw common themes between the participants. RESULTS: We identified themes consistent with the theoretical basis of the CHEST intervention. Barriers to consultation identified in the CHEST Australia trial participants were smoker stigmatization, guilt, fatalism and symptom normalization. We identified a general perceived mistrust of GPs based on previous negative experiences of visiting their GP in relation to their smoking. The intervention tackled barriers around lecturing and feelings of guilt and stigma related to smoking. We identified expected effects on salience and personal relevance of symptoms. Participants reported a clearer understanding of what to look out for and when to take action after the CHEST intervention. CONCLUSIONS: These findings suggest that the CHEST Australia intervention is achieving the desired objectives at the qualitative level through the proposed theoretical mechanisms.