At Model castings it all depends on what the casting is for,- what type of job, a print ad, catalog, showroom, editorial for a magazine?
For product ads typically you will interact/model a mock up of the product for the campaign or print job and the casting director/photographer/art director, who ever is holding casting, will take a photo of you holding using the product, sometimes you just smile at camera and give them your compcard ok thanks, next! Or you are given a breakdown at the casting of what the job involves, what the concept for the ad is.
For showroom modeling you will try on clothing, the shoes, the garments, and the designer could be there and you will try on the items for fit. You may be measured, your photo may be taken in the garment, shoe, etc, and you will leave your comp card.
Being able to wing it in all situations can help when it comes to going on a diverse amount of castings.
It is good to show up early, on time, and not late. Usually you sign in a sign in sheet with your name, agency name, and the agencies phone number, and then sometimes you get assigned a number based on numberical order, #12, #13 or whatever. And you wait ur turn in a waiting area. Usually all the other models stare at each other, sizing eachother up, lol. :) Sometimes during this waiting time, I have ran to the bathroom to change (if the casting is for swimwear or whatever), lotion my body, touch up my face, whatever, and I prepare for the casting.
Then you go in casting room area.
If the casting is for a TV commercial (and sometimes for print work) often are asked to "slate" (speak to a video camera) your name, and agency that sent u name, and hold ur -paper with the number on it that was give to you. Then you will be asked to hold product, cell phone or whatever, and honestly how you are at that instant can reflect if u get the job. It is like a quick interview of "if you are what the client" is looking for, print ads, etc.
If it is a casting for a magazine usually your photo will be taken at the mag office, and if it is for a certain editorial, beauty, shoes, swimwear, etc, that casting will be focused on that type of shot, so be prepared for close-ups, or to have photographed for whatever the casting is for. You will be asked for your comp card, and ahead of time I usually write on my comp card with a Sharpie the agency's phone number that sent me to the casting so I am ready to hand off the comp card asap. Castings are quick, when your name is called usually you are in and out, although you might wait a little while in waiting area.
Say it's a casting at a magazine for a shoe editorial on fall shoes. Fall suggests boots, (your agency should tell you what the casting is for and what to wear), so you should wear nude pantyhose, or tights, depending on what your agency suggests, (and ask your agency questions if you are unsure!) be prepared to try on boots quickly, and show off the shoe naturally to the editor of the magazine. Your photo wearing the shoes will be taken, you will give your comp card to the editor and always say thank you and nice to meet you and be nice and professional.
ASK. If you do not know what to wear, what the casting is for. ASK. It is so annoying when girls so up to a casting and end up asking the other models "What is this casting for?" Um, hello, you should know before you show up so you can better prepare.
Side Note: In NYC there are many casting studios, (where people can book a room for a casting), or for many magazine jobs in NYC it means going to CondeNast, Hearst building, etc, and it's good to know what is the big umbrella of the mag or company you are visiting, and u can research that stuff. The Agency will tell you where to go (address) what the job is for, how to look an details usually.
A casting about how you photograph with a certain product usually or how well you fit for the editorial needs for a editorial in a magazine, and first impressions do matter so come natural with some knowledge of the brand or magazine you are visiting, and look presentable, brush your hair and look nice for the casting. If possible you should wear something that relates to the casting and your agency might say "casual" or "hip" -and skinny jeans and a top are always an easy choice.
When you look at ads in magazines, or watch commercials, usually the girl in the ad or commercial went to a casting and prob did a simialr pose at the casting actually. Being comfortable in different situations, being quick, bright, having a perspective and photographic eye and understanding what's the goal of the job , the vibe, what are they trying to project is a helpful thing when attending castings :) usually an agency won't send u unless they feel u already have a chance for the job.
Being prepared, not just showing up, is what lands you a job,
Psst. In my recently published book Short Stuff: on the job with an x-small model, I share stories of castings I have attended. :)