Well, I was spared having to come up with something to say, since he was on the phone.  But, I'm sure I would have said something stupid or gushing like "I'm a big fan, and thank you for your work, etc."  Or, "How 'bout them cute cowboys all over town, hunh?"
LOL, back in Cheyenne when I bumped into my favorite PBR bull rider at Frontier Days, I was shocked at how tongue tied I was.  I hardly remember exactly what I said, but it was pretty silly and gushing.  I think what I said was nice, but I really was surprised by how much an encounter like that caused me to not think clearly.  Later on I was kicking myself for not saying certain things when I had the chance.  But, it was still a super cool experience anyway that resulted in a handshake and a pic.
It makes me realize that if I ever bumped into a more major celebrity, or someone I'm super invested in like David Bowie or Jake Gyllenhaal or something... I think I would be a ridiculous mess.   Normally I think I'm a sort of "cool as a cucumber" type person, and I need to interact with lots of different folks in my job (some of whom can be intimidating).  But, man, I think the celebrity thing knocks me for more of a loop than I'd ever expect.