If you are interested in working as an extra / background talent on a TV show or movie, then there are a few do's and don'ts you need to know before walking onto set.
1. Don't wear clothes with visible logos or brand names.
A lot of times, extras / background talent will provide their own outfits for scenes. The casting director should inform you what types of clothes you should wear and give you general guidelines. Those guidelines are meant to be followed! Instead of wearing your sweatshirt with a huge Nike logo, wear a plain sweater or something else that doesn't have the big swoosh on the front.
2. Do walk into extras' holding with a smile on your face.
If you look like you don't want to be there, then people are going to wonder why you applied to be an extra. Smile, you might be on camera!
3. Don't complain about how long you've been sitting around for.
Most of the hours an extra gets paid for are spent in extras' holding, waiting to be called to set. Don't think of it as time you're wasting if you're getting paid! Expect to have long hours, and expect for it to be somewhat inconvenient.
4. Do bring a book, homework, headphones, your phone charger or a pack of cards with you.
Like I said previously, a lot of this job is sitting in extras' holding. Bring something to do! Even if you don't think you're going to be using your phone a lot, bring a phone charger. You never know exactly when they'll release you, and having a dead phone when you leave is not fun. If you're somewhat extroverted and want to make friends, then bring a pack of cards! It'll pass the time, and you'll have provided entertainment for a new group of friends. However, if someone tells you that you need to sit quietly and stop playing cards, obey them.
5. Don't ignore what directors and other people in charge are telling you.
Ignoring what others are saying to you is a great way to get kicked off set and sent home early. If someone gives you a direction, you follow it. If they say, "Don't talk during this shot, just pantomime," then that doesn't mean whispering is OK. Don't talk means zip your lips. A lot of times, directions are given for your safety, and to ensure that shots don't have to be redone 20 times due to extras acting like fools.
6. Don't take pictures on set, and don't talk about the scenes you filmed and who was there.
Most of the time, you will sign a non-disclosure agreement stating that you will not release any information about what you've seen / done while you were on set. This is a legally binding contract, and you will get in trouble if you break it. Don't spoil a movie for the people who will be watching it when it comes to theaters. If you want to make sure you're not tempted while you're on set, leave your phone in extras' holding.
7. Don't complain about your pay.
Dude. If you're getting paid to work as an extra, you're getting paid. First of all, it's money, why would you complain about money? Second, you're getting paid to be in a film / TV show. This is as close to being a paid actor / actress as most of us will get. Be grateful for the opportunity!
8. Do thank the people who are in charge of you.
People at hair / makeup, people at costuming, the people who check you in...they're making sure you're going to be the best extra out there. So thank them. It'll mean a lot, and it'll show that you care that they have taken the time to help a bunch of background talent.
9. Don't accept a position as an extra and then not show up / leave early.
This is a job. If you accept a position, then you need to show up for work. If you don't, then you will likely be put on that casting director's black list and won't ever be offered the opportunity again. Also, leaving early looks really bad on your part. If you said you would be there for a certain number of hours, then you should keep that promise. Unless there is a humongous emergency, stay the whole time. Don't leave because you're bored. Once again, if you leave without a legitimate excuse, you will be black listed. You also might not get paid. Be professional.
10. Don't fangirl over the famous people on set and don't say inappropriate things to them.
Each time I've worked as an extra, I've been astounded by the lack of professionalism demonstrated by some of the extras I've worked with. First of all, you are getting paid to work, not to bombard an actor / actress and ask for your own personal meet and greet. They're doing their job too, and it's impolite to interrupt their time. If they approach you and talk to you, then I'd say asking for a picture might be appropriate. But do not interrupt them as they are talking to a director and ask to take a selfie. Most importantly, if you do get to talk to them, do not hit on them. Again, I've been astounded at some of the highly inappropriate things extras have said to actors / actresses they admire. Technically, this is a job, and you could be written up for harassment. Do not slip them your phone number, email address, home address or hotel room key.
11. Do have fun!
This is the most important tip I have. Clearly, you applied to this because you thought it would be a cool opportunity. Enjoy what you're doing! You probably won't get an IMDB credit, but you'll have done something most people don't get to do. Plus, if you're visible on screen, it'll make for a really cool story when you go see the movie with your friends.