If I was dragged up to them and introduced, I’d probably say something like, “Hi, I’m Sunspace, pleased to meet you.” If they were famous artists or someone in my field of work or otherwise doing something similar to what I do, I’d probably add, “I enjoy your work”.
Honestly, I can think of very few famous people I’d make a fuss over. Partly this is because I don’t follow celebrity, and usually have to work to remember peoples’ names and appearances. Most actors mentioned in this thread I wouldn’t recognise, for example.
But even for people I do recognise, there’s probably not that much I’d want to say to them anyways. As DoctorJ said, “If you also do what the artist in question does, it’s usually not a great idea to bring it up (at least not right away). For one thing, that’s his job, and he might want to get away from it. For another, he most likely does it on a whole other level than you do…” The best thing would seem to be to talk about other things than work.
The only time I met a ‘celebrity’ (someone whose music I liked, and whose music had been a commercial success in my circles), we spent an afternoon with the mutual acquaintance who introduced us just talking about… stuff.