Indonesia has claimed a regional leader position in the Association of Southeast Asian Nations (ASEAN) ever since the organisation's establishment in 1967. In practice, however, Indonesia's behaviour in the organisation has varied and especially so since democratisation in 1998-99. Why does Indonesia's sometimes lead, sometimes follow, sometimes obstruct, and sometimes avoid in its engagements with ASEAN? There are two theoretical approaches to explaining Indonesia's foreign policy behaviour in ASEAN. Neither adequately explains this research puzzle. The dominant approach argues the drivers of Indonesian foreign policy are a sense of entitlement and a sense of weakness. This literature maintains that this outlook has remained consistent since Indonesia gained its independence, explaining stability and continuity, but is struggling to explain change and variation. The second body of scholarship points to ASEAN's institutional environment and the limitations it supposedly places on Indonesia's agency. This literature may explain why Indonesia's leadership in ASEAN is not consistently translated into practice. But it does not well explain why Indonesia spends diplomatic capital trying to lead on some issues but does not on others. In the absence of a good explanation for why Indonesia sometimes leads and sometimes does not, this thesis proposes a new theoretical framework. Drawing on role theory, it suggests that domestic level role interactions are the key to understanding variations in Indonesia's leadership in ASEAN. [...]