Whether it's a fully integrated house filled with interactive devices or a single component within an environment, there are three main pieces to creating an intelligent reactive device. First, it has to be able to detect what is going on. We humans use our senses to gather information about the environment around us and the others who may be in it. Sensors do the same thing for electronics, though a sensor may have a narrower scope of stimuli it can detect.
The interactive environment must also have some sort of processing unit that interprets the data gathered by sensors. This is the brain of the interactive environment. It stores user profiles and matches them with preferences. When a sensor detects that a specific person has entered the environment, this processing unit determines the next course of action.
The final piece is some sort of actuator, switch or setting that the processing unit engages to change the environment to best suit the user's needs. This might be a thermostat setting, a sound system or even a haptic feedback system that alerts you to specific conditions.
An interactive environment could have a central processing unit through which all components operate. This would allow for a single point of operation. All the data gathered by sensors would pass through the processing unit, which would send out commands to adjust the environment as needed.
Another approach is to use multiple, independent systems within an environment. This means that you might have an intelligent thermostat and an intelligent sound system but the two aren't connected to each other. One potential advantage of this approach is that if one component's processing unit fails, the others should still work without a problem. Interactive environments may also use a combination of systems with some integrating with a central unit while others are independent.