I would probably stare as I tried to compose my thoughts. Then I would WANT to say something simple, sane and collected, such as "I enjoy your work." In order to not go fangirl on him, I probably would limit myself to "Hi," and nod, and walk on. Then perhaps squee a little bit to myself.
It's kind of like where I work. I work at a Caribou Coffee in souteast Michigan. here, NHL hockey is a bit of a big deal. One of the Red Wings is a regular at my store. I admit that the first time I saw him, I got kind of stupid. But he's not all that talkative, and I figured "You know what? This guy does not need to be stopping for a cup of joe and have the Coffee Wench bugging him about his job." So now, on the rare occasion that I'm working during the time that he comes in, I get his usual, say "hey, mister!" like I do to all my regulars (the ladies get "hey, lady!"), ask if he's getting a paper and be done with it.