TAPA is dead right — almost everybody in the Industry is happy to to talk about what they do (witness all the Industry blogs out there), and most are willing to answer sincere questions if asked at the right time.
Just use your common sense — don’t ask questions when the person you want to talk to is hard at work. Wait until they’re done with the task at hand and are clearly not busy, then ask away. Directors are hard to approach because they’re always working — if not actually directing a shot/scene, then they’re thinking about the next one on the schedule.
AD’s are also very busy until wrap, but the technical people (grip, electric, camera, set dressing, props) have relatively well – defined work/not-work rhythms. After you’ve been on set for a while, you’ll pick up on what’s going on. Then pick your spots, be polite, respectful, and sincere — and don’t forget to smile and say “thanks.”
Do that, and you’ll open up a main line of hard-earned information that may or may not prove useful to you down the line. But at this stage — being a PA — all information is useful, if only to make you more savvy about what’s really going on at any given moment on set.
The one thing I’ll add is that it’s sometimes hard to know when to approach writers. In their offices, goofing off and working hard look very similar. In fact, they are similar. You never know if you’re interrupting vanity googling, or deep meditation on the souls of the characters.
Although, it is probably vanity googling.