You did the pre-read. You did the callback. You maybe even did a second or third callback or some other flavor of bonus round. And then, maybe an hour later or maybe three weeks later, your agent (or manager, or the casting director) calls and says IT'S YOU!
Now what?
First, let me admit that I skipped a step. You may first be "pinned" or put "on avail." It's also common for your agent to say something like, "You're the choice, but they can't book you yet." The thing is, bookings are date-specific, and once you're "booked," they have to pay you... So they often wait because they don't know what date(s) you'll be working yet, or they haven't yet nailed down another member of the cast, so they're not committing to anything until that happens. At this point, they could still change their minds, but most of the time, it's just a matter of waiting for it to become official. So let's be optimistic and say that all works out and you're booked! Hooray!
We've talked before about the fact that it is not your agent's job to get you auditions. So what IS your agent's job? To negotiate your contract when you book something. And that's what happens next. Depending on your relationship with your agent and where you're at in your career, your agent may call, tell you what the project is offering — which can include pay, credit, travel, trailer size, accommodations for nudity or other extenuating circumstances, and other details — and ask if you accept. Alternatively, they may call and say, "this is what you're getting." (If your agent does the latter and you want the former, that's an important conversation to have.) For roles that aren't big leads or series regulars, you'll probably see your contract for the first time when you get to set, so be sure to read it over and make sure everything's correct before you sign. And bring two forms of ID, or a Passport, which counts for both.
Next, you'll probably get a phone call from the costume designer to schedule a fitting. They'll ask for your sizes. Tell the truth. You'll only hurt yourself by fibbing at this point. They may ask you what you wore to the audition, and even to bring it to your fitting or to the set. They may NOT ask you a few things that you should tell them anyway; your coloring (hair/eyes/skin tone, so they can choose clothing colors that will work for you), and whether you have any restrictions in terms of jewelry. (I got my ears pierced late in life, and before I did, I always arrived at my trailer to discover pierced earrings.) After casting and any decision-makers present for your audition, the costumers are the first members of the production team that you'll meet. BE NICE TO THEM. If you're a jerk to the costumers, word will get around, and you'll have earned a bad reputation before you even arrive to set.
You'll start to get emails from the script supervisor containing the script and daily revised pages. (Pro tip... Do not "reply all" to this email. Your question or polite note of thanks will go to every department head on the project.) A day or so before you work, you'll get a call or email from someone on the AD (Assistant Director) staff, usually a 2nd AD, telling you your call time and where you're going. This is another very important person to be nice to. Note his or her name and number, as s/he is the person you'll check in with when you arrive on set, or call if you're late, lost, or have questions.
When you arrive, you'll be shown to your trailer, and sent to hair and makeup. The hair and makeup people know everything. Whether things are on schedule, what mood the director is in, what to order for lunch... EVERYTHING. And they're in charge of what you look like. Be nice to them too. (Are you noticing a trend?)
You may go right to set to rehearse, but most of the time, you'll sit in your trailer for a few hours. Bring a book. If you fall asleep, don't mess up your hair. If you decide to go for a walk, tell someone where you're going. This part of the job is deceptive; it's more than just waiting. An AD could knock on your door at any moment to tell you they're ready for you, so it's important to manage your energy level so you don't burn yourself out by being too excited for hours on end, but you also don't get so relaxed as to be lethargic.
Eventually, someone will come to bring you to set. You'll rehearse a few times to establish blocking, the crew will watch a rehearsal to see what needs to be done for the shot, you'll step away for 10-30 minutes while the "2nd team" stands in for you as the crew gets all of the equipment set up, and then you'll shoot. You'll go through some version of that process for each shot and angle the director needs. If you have more than one scene, you'll do it for each. If you're working more than one day, you'll experience the joy of going back to set and already knowing people's names.
And then, eventually, you'll be wrapped. The crew might clap for you, they might not. You might get home and feel some post-job depression for a few days. It's normal. Once it passes, focus on letting your contacts, targets, and fans know about the work you did, updating your print and online resumes, and getting ready to update your various websites and profiles with clips of the work once it airs.
And in there somewhere, remember to find a few quiet moments to be grateful for being paid to do what you love, and to congratulate yourself on being a working actor!