When people started telling me Spielberg-is-a-jerk stories, that shocked me the most. Growing up, I always pictured him and Michael Eisner (since he hosted the Disney Sunday Movie) as really great, wonderful men.
Then you move out here and find out that, as the quote goes, you have to stab a lot of people in the back to stay on top no matter how nice a guy you are.
It always shocks me when I like a celebrity, too. Kevin Nealon was hilarious, Jet Li was just amazing to be around, Angelina Jolie was just really cool and didn't exude any of the sleb crap she's associated with, Hubert Selby Jr. on the "Requiem for a Dream" junket was fucking cool as hell, Denise Richards is a lot sharper than people give her credit for being...
...but that's about it with a few exceptions.
Other surprises:
Julia Roberts is gorgeous in person, though I never found her all that attractive on screen.
With so many SHORT actors looking tall on screen, it's interesting to meet a taller actor like John Travolta or especially Jim Carrey.
Meeting celebrated directors and finding them to be whiny, art-y types who have been held up by good producers, editors, and screenwriters for so long.
Then meeting directors who turn out to be the nicest, most gracious guys around, ie. Steven Soderbergh, Joe Dante, John Landis, Guillermo Del Toro, Frank Darabont, and others.
Finding out that somebody really is shy as hell and that it's not just an "I don't talk to the press"-thing.
The biggest shocker is when you find out somebody's actually a really sharp cookie no matter what the press might've said about them or the image they're trying to cultivate.