Advancing Social-Communication and Play (ASAP) is a classroom-based intervention targeting the social-communication and play development of preschool-age children with autism spectrum disorder. We used a multimethod approach to measuring intervention fidelity, including adherence, exposure, treatment differentiation, and quality of treatment delivery. Overall, teachers in the ASAP classrooms increased their adherence to the ASAP intervention compared with business-as-usual teachers. Teacher characteristics did not affect the ability to implement ASAP with fidelity. There were no group differences in coded video instructional quality scores. Differences in child engagement between the ASAP and business-as-usual groups were not mediated by fidelity adherence scores, contrary to our hypothesis. Finally, end-of-year summary of treatment delivery ratings did not align with adherence scores. Continued research regarding fidelity of school-based interventions should focus on distinguishing active ingredients from general instructional practices and learning from teachers about the child-level factors they consider when implementing an intervention.