A lot of the time (3-5 times a month) when I'm walking in the street or sitting on the public transit random strangers will make eye contact with me for more than 3 seconds and also smile at me in a way as if they want me to talk to them or something. In all of these situations I try to smile back while walking (or while sitting if I'm sitting) and then continue on my way (or look the other way) but sometimes I feel like I want to get to know the random stranger too as they have expressed interest. To clarify, I'm talking about situations where it's more than just a quick glance and smile, but a prolonged one that may include other things too such as turning to face me. What can I do in order to do this successfully without it being awkward or intimidating the other person in the off case that they didn't want me to approach them, and also to befriend the individual in the process if they do?
edit: M/20