SAGAFTRA is the union for actors. This covers film, television & voice over.
There are three primary ways into the union.
#1. Book a role on a union project. This covers anything that is credited at the end crawl and has lines or significance.
#2. Book a union commercial
3#. SAGAFTRA vouchers
Lets cover these in detail so that you can understand a few things. Production gets a monetary penalty for hiring a non-union person for a union role. So, you have to be really worth it for production. For the most part production only hires non-union for union roles for the following reasons.
a) A relative of someone. Nepotism is likely in the top percent of union eligibility of individuals above the age of 18.
b) The role is extremely specific. If a production needs a 500lb person that is willing to be ridiculed in a public format, they do not care about the fee. It will be hard enough finding the individual that fits for the role.
c) Children. Production only cares if the kid fits the part and can hold their own. They prefer union, but if the talent pool and availability isn't there production will pay the fees. I am not 100% certain, but I do believe that the fees for youth are also lower than those above 18. I am too lazy to crawl through the tombs of legal speak to give you absolute certainty, but somewhere along the way I was told that and never verified it.
d) Someone already a name in another field. This is the singer, the sports figure or the politician or some other socially relevant figure that production is willing to pay the fees for.
e) Time factors. This means that casting needs to fill a role rapidly and the union members they saw either have scheduling conflicts, want too much money or rejected the role and you are the next best option. I would love to tell you it is because you are the best for the role. That does happen, but honestly the money and politics usually win over green talent.
Perhaps the most common way into the union is booking a commercial. Where TV and film productions have a penalty for hiring non-union, commercial does not have that penalty. They have more paperwork, but that is it. I don't know what the actual percentage is, but by casual survey those entering acting professionally over the age of 18, commercials would be well over half of how actors got into the union.
The last way into the union is background work. You need to gather three vouchers for playing background on a union project within a certain amount of time. This is tricker than you may think. A voucher is what they call the union time sheet for background. It is like a time card. A copy goes to you, production accountant and another to SAGAFTRA. Production gets financial penalty for handing out that last voucher that made a non-union person SAGAFTRA eligible. So, if you are going to play this game, never admit that you have any vouchers when they ask. But you need to be aware of something here. Background lessens your position. Sure, if you have never ever been on set you can learn things from being in the environment. But, you are also putting yourself in a box. I know many many actors that have worked for years doing background work and it never amounted to anything. In fact, out of 25 years of observation, those who have done background work to get their union cards stayed in background, left the industry or after many years booked a small role in an indie short film that never got them anywhere. Statistically speaking I have never met a single individual who ever made a career this way. Every agent and every manager in the industry will say not to do it. The ones that ignore that are not looking at futures. They are closing the door on themselves by lessening their value all because they think being on set is exciting. It's bad positioning for you. It's like wanting to go to a university and since you don't have the grades or money to get in you become the janitor thinking that you will get educated that way.
Also note these factors:
Odds on available vouchers are seriously low. Production has to hire a certain number of union people for background. The only reason a voucher becomes available is if one of those union background players is a no show. That is an extremely rare occurrence. If a voucher is available then the person who the 2nd or 3rd AD likes the most is the one that is going to get that voucher. On the TV series that my kid was on, over a 3 year period I think maybe 2 or 3 vouchers went into non-union hands because of no shows. Our 3rd AD was very fair on who he gave those to, but is was to his discretion and if he accidentally gives a 3rd voucher to someone he could loose his job. Especially if that happens too often.
Background pay sucks. I mean it really really sucks. So if you are a "starving artist" wanting to get ito the union and you think to yourself "I will do background work" you better be really happy with eating top ramen and have 10 roommates, cause you will not be able to pay your bills doing back ground work.
There is a reason background has a bad rap. The craziest people in the industry are background. I actually mean that. There are halfway houses that drive people just out of rehab or just out of jail to register with Central asting (the largest background casting house) because they do not do background checks and they require nothing as far as skills. On set you have the desperate wannabes doing random things they think would be good for them. Mostly it is just creepy. Case and point, on my kids TV series there was a regular background player. He began stalking the kids instagrams. He showed up to every event that Jade did. We don't even know how he knew about them, because some were not advertised. He would show up at screenings and interrupt Jade talking to the director or to a media person and he would introduce himself saying "I work with Jade on School of Rock" as if he is another cast mate. Jade was not the only cast mate he did this with. He would bring gifts to the girl cast mates. Things like jackets and bracelets and "handmade" art work. But coming from a 20 year old, this concerned the parents to a point that we had to speak with production and that individual was never on School of Rock again. There was also the girl that was a regular background who got fired from the show because during breaks her and her mom would walk into random offices on the Paramount lot passing out headshots and resumes thinking that this was going to be a good marketing technique. What this did was ban them from the Paramount lot and get them blacklisted from every casting director on the lot. The list goes on and on. These are two small examples, but nowhere near what production saw and experienced with background. But it is an example of the reputation background gets. The craziest people do background and you are positioning yourself in with the crazy ones every time you do background work. Stick with holding your integrity and getting your union card the way that you should. By working.