INTRODUCTION: Despite widespread calls to end violence against women, there remains limited evidence on how to prevent it. Community-level programmes seek to engage all levels of the community in changing norms that drive intimate partner violence (IPV). However, little is known about what predisposes ordinary people to become active in violence prevention. METHODS: Using data from the SASA! study, a cluster randomised trial of a community mobilisation intervention in Kampala, Uganda, we explore which community members are most likely to intervene when they witness IPV. A cross-sectional survey of community members (18-49 years) was conducted 4 years after intervention implementation began (2012). Among those who had seen IPV in their community (past year), multivariate logistic regression, disaggregated by sex and trial arm, explored the associations between 'trying to help' and demographics, IPV experience (women)/perpetration (men), childhood abuse experiences, IPV attitudes and SASA! exposure. RESULTS: Overall, SASA! community members were more likely to intervene than their control counterparts (57% vs 31%). In control communities, older age (women), increasing relationship duration (men), talking to neighbours (men) and believing it is okay for a woman to tell if she is experiencing IPV (men) were positively associated with trying to help. In SASA! communities associated factors were increasing relationship duration (women/men), employment (women), talking to neighbours (women), childhood abuse experiences (women), lifetime IPV (women/men), IPV-related attitudes (women/men) and greater SASA! exposure (women/men). CONCLUSIONS: Differing results between intervention and control communities suggest contextual factors may modify the effects of personal characteristics/experiences on helping behaviours. Motivation to act brought about by personal experiences of IPV, for example, might only propel individuals into action if they are equipped with the skills, confidence and support of others to do so. Community mobilisation can help create environments and synergies supportive of action. TRIAL REGISTRATION NUMBER: NCT00790959. STUDY PROTOCOL: Available at http://www.trialsjournal.com/content/13/1/96.