I have only seen one celebrity in person so far: it was Casey Neistat, one of the biggest YouTubers in recent years. I was with my friend in San Francisco going clothes shopping and we saw him with his son, my friend and I waved at him but didn't go ask for a picture because we didn't want to interrupt his time with his son. Honestly though there are only a small handful of people I would ask for a picture and those are really just my favorite musicians or a few actors. It really just comes down to the situation I think, I know some celebrities hate pictures and if I knew a celebrity I like doesn't like pictures I wouldn't even ask, but if they don't appear to be in a huge rush or having a moment like Casey Neistat was with his son (he was visiting his son from New York so it's not like they're always together so we left them alone) then I would probably consider respectfully asking for a picture and if they say no then oh well. I guess there was one other time, I was at San Francisco at the Bell Graham Civic Auditorium for the League of Legends World Championship (group stage, not the championship game, but I did go to that too in LA) and when I was walking to the bathroom I saw one of the casters for the games walking and nobody else was around him and he was my favorite caster so I told him I loved his commentary and asked for a picture and he said yes and seemed happy to take the picture with me, he was super cool about it. Obviously we were at an event of mutual interest so it wasn't randomly in the street, but still my one other interaction with a celebrity which I just realized funnily enough they're both internet celebrities.