We are interested in developing real-time applications such as games or virtual prototyping that take advantage of the user full-body input to control a wide range of entities, from a self-similar avatar to any type of animated characters, including virtual humanoids with differing size and proportions. The key issue is, as always in real-time interactions, to identify the key factors that should get computational resources for ensuring the best user interaction efficiency. For this reason we first recall the definition and scope of such essential terms as immersion and presence, while clarifying the confusion existing in the fields of Virtual Reality and Games. This is done in conjunction with a short literature survey relating our interaction efficiency goal to key inspirations and findings from the field of Action Neuroscience. We then briefly describe our full-body real-time postural control with proactive local collision avoidance. The concept of obstacle spherification is introduced both to reduce local minima and to decrease the user cognitive task while interacting in complex environments. Finally we stress the interest of the egocentric environment scaling so that the user egocentric space matches the one of a height-differing controlled avatar.