Y'know, I always gotta feel bad when fans go running up to celebrities, asking for autographs and pictures. Being a Celebrity is a double-edged sword and a lot of times it tends to wear on the folks.
Personally, if I saw Robin Williams or any other celebrity of any kind, I'd be definitely have a big stupid grin on my face, but I'd try to be very cool about it and treat them like any ol' acquaintance. They come to game stores and what not to relax and kinda be a normal folk. If they want a game or chill at my table, I'll be giddy as hell. If not, eh. I'm not going to drag my seat over.
Of course, it's when I get home do I start giggling like a school girl and rambling at everyone I know will keep their mouth shut about how
frickin' sweet it was to play/paint with/be around/share the same air as said celebrity.