Walk up RP (not that I seem to ever be able to get involved in any) is often the most fun. It is an open world and if people are playing in Say in a public setting, including far flung locales that you just so happen to stumble upon, the RPers should anticipate potential "interruptions". Of course, use your judgement. If its a date or a conversation that seems to be tailored specifically to the people involved maybe wait to approach them at another time.
Everyone is different. If you try to get involved in a situation and it becomes clear that your interaction isn't welcome just apologize (either IC or OOC or both) and move on.
Good luck.