Are there any more annoying words than 'Please may I borrow your pen?' There you are, writing down something important and someone, who can't be bothered to carry a pen or pencil, wants you to interrupt your chain of thought and wait while they use your property. They'll probably need reminding to hand it back to you, as well. This is just one example of things you should always have with you when rehearsing or shooting a film. The Words of Wisdom are Always run a checklist before you leave home. That way you make sure that you don't forget anything, The items on your checklist will include your script, the schedule if one has been issued, some water, something to read while you're waiting, some cash (you might want to buy someone a drink at the end of the day.) You will of course have checked the journey so that you get there is good time. And you'll be bringing a pen or pencil - and maybe a spare, in case you find yourself sitting next to an idiot.
The next Words are Maintain Continuity. Once you have been cast in a part don't change your hairstyle or any other feature of your appearance without permission. If you are using any of your own clothes and accessories don't make any changes. It's not a good idea to drink alcohol at a lunch break. Even just one drink changes the way you speak. Similarly, the location catering may be delicious, but eat sparingly.
Get to Know Who Does What. If it's a big production the number of people involved is quite staggering and may be baffling to a beginner. It's worth studying the Wikipedia article on Film Crew, which gives you a rundown of who does what. On arrival you will probably report to the '2nd AD' (second assistant director) or possibly a 'PA' (production assistant.) whose job it is to shepherd you through costume (sometimes known as 'wardrobe'), hair and make-up and deliver you on set when required. Remember that everyone is there to help you and if you keep your eyes and ears open you will learn a lot. You may find you are working with a big star. If so, don't stare, don't behave like a fan and, above all, don't ask for an autograph. That would not be appreciated. It is more likely, of course, that your first job will be on a much smaller scale production. In which case there may be a lot of overlapping between cast and crew and you may be asked to muck in and help out in all kinds of ways. You will, of course, show willing - within reason.
The next Words of Wisdom: Trust the Director. The story is told of Marilyn Monroe shooting the film Bus Stop, with her acting coach, Paula Strasberg, standing next to the camera. Before and after each take the two of them would ignore the director and go into a huddle and have a whispered conversation. What this did to the cast and crew can only be imagined, but sometimes as many as thirty takes would be required for each shot. Coupled with Marilyn's notorious unpunctuality the result was that filming fell more and more behind. That's an example of not trusting the director. You must trust the director. Even experienced actors find it hard to visualise the composition of a shot while they're performing. You don't get to see through the lens. You don't know how the scene is going to be edited. You don't know how music is going to be used in it, let alone CGI (Computer-generated imagery.) The director is carrying all these in his or her mind so Trust the Director.
If you're a stand-in you're there to help. You may be asked to be a stand-in for an actor who is temporarily unavailable. You are of similar stature and you are there to give another actor an eyeline and someone to play off. So don't just stand there like a plank! Match your facials and body language to the person you are substituting for. That will help your fellow actor.
And finally: Learn to Hit Your Mark Without Looking at it. Sometimes you have to move into shot and, because it's a close-up or an intricately composed shot, you have to stop at a precise position, usually marked on the floor by a piece of 'gaffer tape.' (The 'gaffer' in a film crew is the chief electrician, incidentally.) But you mustn't look down while you're doing it. The audience don't want to see an actor looking for where his feet should be. So how do you do this? Practise! At home, put a piece of tape on the floor and go and stand over it, so that you became aware of where your surroundings are when you're on your mark. Then go to your starting position. Look at your mark and visualise, as footprints, the steps you are going to make to hit your mark. Then, without looking down, walk those steps. See how close you can get. Then keep on practicing, trying different distances and angles. After a while you'll hit your mark every time and you won't be the one who gets it wrong.