An interesting phrase I heard once said “the observer changes the observed.” It made sense, because people ignore what’s happening around them.
I saw a woman in a grocery store slapping and yelling at a small child. So I stopped, stood still, and stared until she saw me looking. Something in her face changed, and she bent down to talk to the child calmly.
I do that sort of thing often now. Because when people see someone looking AT them instead of looking away, it often has an effect.