This study augmented the Prototype Willingness Model (PWM) to assess reactive and deliberative decision-making underpinning bystander intervention in gender-based violence contexts. There were 2079 participants (50% male, 49% female, and 1% unreported), aged 11–15 years old (M = 12.32, SD = 0.91), attending 19 secondary schools across Scotland. Participants self-reported the augmented PWM variables, then their intervention behaviour approximately 1 month later. Path analyses mostly supported the predicted relationships between positive and negative bidimensional attitudes, subjective norms, prototype perceptions, perceived behavioural control, and self-efficacy on intentions and willingness. Willingness predicted positive (speaking with a teacher) and negative (doing nothing) intervention in less serious violence. Self-efficacy predicted negative intervention in more serious violence. Subjective norms positively moderated the attitudes–intentions relationship. Overall, the results suggested that reactive (willingness) more so than deliberative (intention) decision-making account for intervention when young people witness gender-based violence. Additionally, the findings highlight the complexity of bystander intervention decision-making, where adding control perceptions, bidimensional attitudes, and moderators have independent contributions. Furthermore, self-comparison to the typical bystander who positively intervenes (prototype perceptions) was the strongest predictor of intentions and willingness, highlighting in a novel way the importance of image and group membership on decision-making.