You pretty much how to discover the talk option in some of the older games. When 2 characters walk next to each other, a talk option can appear. It has nothing to do with supports. Some of them just appear based on the chapter you’re in and who you bring. It’s no big lost considering most of the new characters get plenty of development in the sequel. They just didn’t bother to focus on the returning ones. The Radiant Dawn has about 72 party members total.