Manifestations of insistence on sameness (IS) and circumscribed interests (CI) are complex, with individuals varying considerably, not only in the types of behaviours they express, but also in terms of a behaviour's frequency, intensity, trajectory, adaptive benefits, and impacts. However, current quantitative RRB instruments capture only certain aspects of these behaviours (e.g., mostly frequency or general "severity"). Thus, the current study utilised a semi-structured caregiver interview to provide an in-depth, qualitative characterization of different aspects of IS and CI presentation. Caregivers of 27 autistic children and adolescents displaying IS and/or CI behaviours (3-16 years; 18 males; 9 females) participated in a semi-structured interview. Responses were analysed using thematic framework analysis. Framework analysis identified nine different aspects of IS and CI presentation: (1) intensity, (2) frequency, (3) emergence of behaviour, (4) changes over time, (5) day-to-day fluctuations, (6) purpose/adaptive benefit, (7) experiences of distress, (8) challenges for the individual, their family, and their socialisation, and (9) management strategies and their effectiveness. Autistic children and adolescents were reported to vary greatly on each of these dimensions. Findings demonstrate the complexity of IS and CI presentations and highlight the need for more comprehensive quantitative assessments that independently assess the frequency, intensity, and impact of behaviours. Further, findings reported here emphasize the need for ecologically valid measures that assess the contexts in which these behaviours occur and how their presentations can change within and across days.