Once we have built a model that is able to mimic a human face, we can then think about modelling behaviour of interaction between two persons. We can for instance study how the parameters of the two persons vary together by computing their joint probabilities. We can also study the correlation between each pair of variables that make the model.
Another possibility is to study the reaction from one person to the other one. Questions such as "How long does the reaction take ?" or "How strong is the response ?" may be answered. It may allows us to build a model of interaction that takes these elements into account.