The problem these days, is that 75% of the people who who look like they are talking to invisible people on the street are wearing earbud phones. You do not always know WHICH 75%.
I often see people walking and talking with their heads down, making no eye contact with passerbys. When I see this, I tell my wife that cell phones have normalized insanity.
Sometimes it's pretty obvious. I was waiting at a traffic light a while back and I watched this guy at a bus stop arguing with an invisible person. The argument escalated into a shouting match, until the guy started walking away. Apparently the invisible party said something really offensive at that point because the guy rushed back and dove over the bus stop bench to strangle the invisible person. After that the light changed and I didn't see how it all turned out.
I'll never forget the time 20 years ago, when my wife and I stopped for lunch at a Mc Donalds in Minden, Nevada. After ordering inside, we turned to look at a man shouting to himself and wildly waving his arms around as he spun in circles. At first we thought he was a lunatic until I spotted the plastic earbud and long thin microphone protruding from it. First time we'd ever seen someone use a cell phone like that in public.