The person most actors will deal with on set is the AD or Assistant Director. Big sets will have several ADs (known conveniently as First AD, Second AD and so on).
An ADs job is all encompassing but generally involves getting the actors organised and ready to shoot, and making sure the director’s wishes are carried out to the letter. Various levels of AD will also be seen herding extras about, checking that the catering is ready, giving out tomorrow’s call sheet to the actors, driving people about, and so on.
Find the AD when you first come onto set and let them know you are there. Then if you have any questions, they’re the ones to ask. Likewise if you need something… anything… then ask an AD and they’ll sort it out for you.
Also – very importantly – make sure to let the AD know if you need to leave the set for a few minutes (to go to the toilet, for example) so that if you’re needed suddenly at least one person knows where you are at all times. Entire sets have come to a grinding halt while the ADs rush about trying to find a missing actor (who was not treated politely when they finally showed up having wandered off to buy some cigarettes without telling anyone!).