This paper presents a rigorous, analytical framework for interactive control methods such as stiffness and impedance control. This paper does not present a novel synthesis method for robot control design. Rather, it presents a proper framework to analyse controllers for robots whose purpose is to interact energetically with the environment. First geometrical tools are introduced that are used in kinematic and dynamic analysis of the spatio-mechanical systems common in robotics. 'Port behaviour' and 'behavioural deviation' are then defined both intuitively and rigorously. The utility of this framework is demonstrated by a non-trivial example. Concepts of the behavioural approach are used.