One of the key issues in the field of virtual agents is the design of agent architectures which fulfill the conditions necessary to manage at the same time real-time reactive behaviors and longer term cognitive abilities. We present in this article FlexMex, a flexible multi-expert meta-architecture for virtual agents. The main challenge lies in the structuration and organization of several modules addressing each a specific type of intelligence, each producing its own desires, goals, plans or motivations for behavior. The meta-architecture presented here is at a level which is largely independent of the module contents. The propagation of the behavior proposal through the architecture down to the level of a final decision step has to be in a flexible and manageable manner. We instantiate our proposal in the context of a projecting aiming at animating autonomous actors living in a virtual city, while respecting constraints of credibility, real-time, scalability and reuse of the architecture.