Street portraiture is hard for me, but has become easier over time. I am not a big fan of long lens or surreptitious shots of people you see in the crowd; you are in some way "taking" their image without establishing any kind of relationship between photographer and subject much less getting their permission to take their photo.
The glimpse of someone interesting-looking is usually momentary. You see them and quickly they are lost in the crowd or context, going on with their business unless you catch up with them and attempt a conversation, often complicated by language inadequacies; trying to reassure them that your interests are, what, "honorable." You try to communicate that you would like to take their photograph. Explaining why is not so easy. You don't want to do a "hit and run" or "drive-by" shot. Often it is someone who has a unique look or is in a context that helps to define them. But sometimes the very context, itself, takes away from portrait by being too cluttered and distracting. Or the light is pretty bad. Sometimes I have been able to talk the person into placing themselves into a better background, better lighting. The portrait has to be shot up close for the most part, so there is this intrusion, that you are imposing yourself into their personal zone. In any event you generally do not have much time. It makes people nervous if you fidget with exposure, focus, composition so you just have to snap away and hope for the best.