My Masters thesis involved using Transtheoretical Model (TTM) of Health Behavior Change constructs in spoken conversation embodied conversational agents to motivate health behavior agent. This was one of the first agents which used Machine Learning to extract TTM constructs and use them in dialogue systems. The TTM is very interesting health behavior change theory. It describes the steps which people tend to follow to change their health behavior. It describes Processes of Change as concrete steps people follow in each of these processes for changing their health behavior.
In short, different stories need to be told to people who are just beginning to lose weight compared to people who are already in a state where they want to maintain their weight. TTM describes these process people follow.