Gender norms serve to normalise gender inequalities and constrain girls' agency. This paper examines how girls' agency, along a continuum, is influenced by the interplay between constraining and enabling influences in the girls' environments. We analyse data from a qualitative study nested within a cluster randomised evaluation of Samata, a multi-layered programme supporting adolescent girls to stay in school and delay marriage in Karnataka, South India. Specifically, we compare agency among 22 girls from intervention communities and 9 girls in control communities using data from the final round of interviews in a qualitative cohort. Using the concept of 'thin' and 'thick' agency on a continuum, we identified shocks like mothers' death or illness, poverty stress, gender norms and poor school performance as thinning influences. Good school examination results; norms in support of education; established educational aspirations; supportive parents, siblings and teachers; and strategic government and Samata resources enabled thicker agency. The intervention programme's effect increased in parallel to the gradient from thin to thicker agency among girls in progressively supportive family contexts. Engagement with the programme was however selective; families adhering to harmful gender norms were not receptive to outreach. In line with diffusion theory, late adopters required additional peer encouragement to change norms.