In this paper we present an intelligent believable agent development environment. This was achieved by connecting dMARS, an agent-oriented system, and Open Inventor, an animation system. We describe the design of this environment which involves a connecting layer and some plan templates for (i) managing the synchronisation between actions determined in the reasoning system, and their execution in the virtual world of the animation; and (ii) managing the physical aspects the agents, including sensing. Emotions and personality are also modelled to some extent. We successfully implemented a real-time animation using this environment which we call Dog World, with dogs which behaved in interesting ways, reasoning and interacting with their environment. The merits of the system are evaluated conceptually and a comparison is made with two other believable agent systems.