In this paper, we demonstrate the use of interactive simulation as a tool for workcell validation and optimization. The proposed technique uses real-time physics simulation to immerse the design engineer or production planner inside a responsive virtual model of the factory. The user can interact with components and tools, as well as with the robots performing their assigned tasks, including collaborative steps. The hand guiding function of sensitive lightweight robots can be simulated using force feedback devices. Future application scenarios can be directly tested in the assembly process simulation. Thanks to this first-person 3D experience, a better understanding of the risks, complexity and potential improvements can be reached. With this knowledge, it is possible in early planning stages to define the first working and protection areas for safety programming. These are necessary for the human-robot collaboration to be safe. In addition to safety, the process reliability can be optimized in the simulation, too.