Secret to it:  really listen hard when you run into someone.  Find a question to ask them (hopefully something interesting like travel, or what their kids are doing, or something).  If you ask a good question, someone will most likely respond, and then the conversation is on from there.  It does amaze me how many will "settle" for just pleasantries and not really get into it.  (No politics or religion, of course).  Some of the more fun moments the couple of HS reunions I have done is finding someone I knew just a little (or only knew OF) but then discovering a way to get into a more involved conversation.  Actually getting to know them better, sometimes for the first time.  Takes a little work, and honestly, it exhausts me a little.  (Maybe there's some Asperger's?)  But I find those kinds of reunions really fun and kind of fulfilling.  

I will say that the last one (2018) was more scary in the beginning.  Having come off a divorce in the interim, I felt like I might be judged as if a bit of a failure on that basis.  I also worried if some single woman not of much interest to me might recognize that and come on too strong or uncomfortably strong.  But that didn't happen.  People were really nice, and we had lively conversations that actually went all over the map.   I ended up being really glad for having taken the "risk" and just gone to it.  Perhaps because it was a really small, semi-rural town, it is the case that most of us knew each other or at least knew of each other.  That and the shared experience of being from an out-of-the-way place makes for more common ground.  It also seems to make people automatically interested in the globe-hopping travel I have had in the last 20 years.  People seem to have heard about that, and they like to ask me about places like . . . Russia.