From reddit, user filmstudiogrunt:
"I work at a major film studio in Texas. On big film sets everything is so chaotic that pretty much anyone can walk on if they pretend like they know what they are doing. Anyone aside from the top ranked people won't hassle you because they don't want to piss someone off that could get them fired. I'm fairly confident that anyone could meet their favorite celebrity simply by buying some food from the store, saying you're a p.a. or crafty, and walking up.
Also security is a complete illusion. Just don't look nervous!"