We propose an interactive dynamic simulator for humanoid robots using constraint-based methods for computing interaction forces with friction. This simulator is a part of a general framework for prototyping called AMELIF and is a successful integration of physical models. We focus on optimizing the computation of the dynamics to obtain real-time simulations allowing multimodal interactivity. Our simulator has been validated in two ways: first by comparing real sensors' measures and simulated values, then through different scenarios of complex manipulation tasks on the HRP-2 humanoid robot, bringing new insights to interactive robotics.