I went to Pittsburg from Cleveland to meet Carroll Spinney (Big Bird, etc) at a toy convention. It wasn’t long before he died. I attended his talk, got an autograph AND a photo (both paid for). I cried and told him how much he meant to me. He couldn’t have been sweeter. I am SO glad I did it.
I also met Brian O’Holloran at the same con. Dante from Clerks. nice fella, I didn’t cry 
Back when there were video stores, a popular (apparently) one in Tampa hosted Tom Savini, who was in the area filming/plugging a vampire movie of his. There is not a nicer human being on the planet. I brought some VHS tapes for him to sign and he autographed my vinyl copy of the “Creepshow” soundtrack. I kept the questioning brief, as the line was growing rapidly, but he seemed to really enjoy talking with fans.
Interesting idea!
When I was on summer break, I was bumming around with my best friend, and he suggested that we stop by a conference, in the hope of meeting Roger Zelazny, who was his favorite author. We talked into the lobby, and Zelazny was walking out a the same time. My friend got completely tongue-tied, managed to shake his hand, and babbled something incoherent. Zelazny smiled, thanked him and waked on.
I thought it was hilarious. It should be noted that my friend was one of the smartest guys I knew, who went on to have a career at the State Department, and could talk intelligently on any subject.
I wonder what he would have done had I said “no.” Maybe he would have offered me spiritual counseling on the spot? Maybe he had a little box gadget that expanded into a confessional?
Not my personal experience, but a few friends went to Dragon Con in Atlanta (back in the 80’s I think?). They wanted to eat lunch, but decided the food at the convention was too pricey, so they went to a Wendy’s down the street. They settled at a table, and a gentleman asked if he could join them. It was James Doohan! He saw their convention badges and knew they’d be kindred spirits.
They said he talked about his dogs. They didn’t ask him anything about Star Trek, and just enjoyed his company.
Wow. It probably wasn’t the same place–when I saw him it was definitely at a con–but I was going to tell a similar story about Don West (or Mark Goddard, to use his real name). I thought about going up to his table, but to my shame, I didn’t.
I did once see Betsy Palmer at a con. Her line was pretty long, so I didn’t join it. If I had, I would have told her that while everyone else there knew her from Friday the 13th, I was perhaps the only guy in the place who remembered her from I’ve Got a Secret.
Edited to add: Building on Knowed_Out’s story, once at Flashback Weekend in Chicago, we spotted Michael Berryman (the strange-looking guy from the original The Hills Have Eyes) having lunch at a food court. He didn’t join us, though.
Joe Haldeman did a reading of a chapter of a new book when I was in college (University of Maryland). The room at the library was full but it wasn’t a very big room. He stayed and answered everyone’s questions as well as signing any books. Very gracious and friendly.
The place I saw him didn’t normally have guests. It seemed really out of place.