This paper proposes a methodology for the development of multimedia systems. It is based on a time-sensitive, reflective actor framework that centers on lightweight actors, non-overkilling concurrency and customizable constraint-directed scheduling. A multimedia system can be visualized as a collection of media actors, i.e., autonomous, concurrent and computational processing entities, involved in multimedia sessions. QoS requirements associated to a multimedia session are incorporated in reflective actors called QoSsynchronizers that manage and enforce application-dependent QoS parameters. Timing QoS, parameters are first specified by using Time Stream Petri Nets and then translated in terms of a QoSsynchronizer. In order to support QoS constraints analysis and validation, media actors and QoSsynchronizers are prototyped under simulation by exploiting a flexibility of the adopted framework to operate transparently under virtual or real time. Thus a seamless transition from the modeling to the implementation stages can be obtained. The paper describes the use of the methodology for the development of live and on-demand multimedia applications.