what does (Cast overview, first billed only) mean?

First billed means top of the list.

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Billing is a term used in advertising.....I think it might come from "handbills" but I'm not sure.....handbills are leaflets/flyers that are handed out on the steet by an employee of a resturant or car dealership....or a movie house.

Film posters or trailers are considered ads, first billed means the actor...usually the big name in the film....is listed at the top, to gain attention....then followed by the other well known actors in the film.

I'm guessing you saw this at IMDB, or something similar....in the "cast overview" sense....they are only showing the actors that are easily recognised by name alone, or that played a significant role in the film....as in the actors that are listed first in the credits.

If you are a real superstar, you get billing on screen before the title of the film comes on.

Thanks each