On one hand you've got static, preauthored dialogue trees, which are seriously limited in the amount of freedom they can provide. On the other, natural language processing (NLP), which would work great if it wasn't all but impossible to implement. Those extremes aren't promising, but there's been plenty of activity in the middle. Façade. Galatea. The Sims. Masq. Siboot. Ultima. Civilization. Quantic Dream. BioWare. Bethesda.