I've only had the opportunity to talk to a few "celebrities" even though they're only Voice Actors, I met Vic Mignogna at Colossal Con 2011, he seemed to be a genuinely nice guy , they were going to cut off the line on the autograph session because time was up since they had a certain amount of time for each autgraph session and he actually stayed after a good 30 minutes or more to finish autographing things for everybody that was in line, my mother however is convinced he's a jerk because he apparently almost ran her over and then denied it.
I also got to meet Todd Haberkorn at Colossal Con 2011, he seemed nice too, though were as Vic was actually moving out from behind the table for pictures and interacting with the fans and having conversations , Todd seemed to not care as much he just kinda leaned over the table and then you had to hurry up and snap the picture.
I also met Rosearik Rikki Simons and his wife at Ohayocon 2010 ( I think it was) , Mr. Simons voices Gir from Invader Zim and he was a genuinely nice person he even put up with the fact that instead of managing to actually reply I most of the time only managed a squeak because I was soo shy and horrible at talking to people at the time, he even asked me where I got my awesome Gir hoodie ( I had on one of the ones from Hot Topic that was the green with the black sleeves and had the hood with the gir ears ) and said my outfit was cute ( I was wearing the hoodie zipped up and had black pants and black shoes on , it was my unintentionaly Gir outfit), al in all he was really sweet, I bought a picture off of him of Gir to get signed and he signed it "Ride the pig Lexy".
I also at Ohayocon "met", well tried to talk to a VA from Fullmetal Alchemist , I think I was the woman who voices Envy , I can't remember for sure , and I asked her something during the panel and she acted like she was p*ssed that she even had to be there let alone actually talk to the fans , though it could have just been she was having a bad day, becasue it was supposed to be a FMA panel with all of the VA's from the show that were going to be attending and it was only her and Aaron Dismuke that actually were there for that panel.
I tried getting an autograph from Aaron Dismuke at Ohayocon but I went home from that convention without an autograph even though he was one of the main reasons I wanted to go in the first place, but the whole con was organized horribly, there would be one panel that was supposedly in one place only to get there and find out that it was actually at the comeplete other end of the building, and the autograph sessions were only 1 a day and only for 15 minutes and they filed the first so many people in and they were sat in seats and then called up by rows and after the doors closed you were pretty much assured to not get an autograph.