A casting is not an event you dress for. The room needs to see your shape and your face, and anything working hard to be interesting gets in the way of both.
Fitted, plain, one colour
Slim jeans or straight trousers, a plain top, nothing oversized. Black or a single muted tone. The client is mentally putting their clothes on you, and every pattern you wear is one more thing they have to subtract.
Shoes you can walk in
Simple heels if you can walk properly in them, flats if you cannot. A walk that is fighting the shoe reads as a walk that cannot be directed, which is the opposite of what you came to show.
They are not deciding whether they like your outfit. They are deciding whether they can see past it.
Face and hair, left alone
Skin clean, hair down and dry, no strong makeup. Bring a hair tie. If you have booked a colour or a cut, do it after the casting, not the morning of, because the client is casting the person in front of them today.
Build the book
The full build: audit, styled studio day, ruthless edit, printed comp cards.