Is there some kind of rational, intelligible pecking order to Hollywood and TV screen credits? Specifically, are there quantitative measurements that apply to the term "star," "costar," "also starring," "guest star," and others? Why, in some TV series, are some actors credited "as" their characters (e.g., Charles Haid "as" Renko in Hill Street Blues), even though others in the same series are not? And are there industry guidelines on type size with respect to job performed?

Apart from indicating in a vague way the relative clout of the players in a given production, the terms "star," "costar," and the like have no intrinsic significance. For instance, while it is usually true that "starring" in a movie means that you play a leading role, one will occasionally see something like "Cecil Adams in The World's Most Fascinating Newspaper Column starring Slug Signorino," in which case the latter personage clearly has a subsidiary, and we might even say a hopelessly insignificant, part. What's important in the movie biz is not the terminology but the order, and in the case of the advertisements the relative size and position, of the actors' names. These things are generally worked out in extraordinary detail well before filming starts in negotiations between the production company and the individual actors' agents.

The type size to be used for each name in a movie ad is usually expressed in percentage terms, with 100 percent being the size of the movie title itself. Big stars often insist on having their names appear at 100 percent (or bigger, if they can swing it, which usually they can't), and it works down from there. If you ever happen to be negotiating some macromazuma deal with Fox you might want to bear in mind the crucial distinction between the "art" title and the "billing box" title. The art title is the name of the movie as it appears in fancy (and often humongous) lettering in the middle of the ad, whereas the billing box, where the title usually appears again, is that indecipherable smudge of print at the bottom of the ad. You want to get the size of your moniker pegged to the size of the art title if possible, but the more common practice is to peg it to the billing box title. So what we've got is all these agents and lawyers and stuff spending their time arguing about something nobody on the entire planet can read. But hey, it's the PRINCIPLE of the thing.

Once you get past the top stars, any number of terms, such as "also starring" and/or "costarring," may be used to designate the supporting or "featured" players. Agents for supporting actors use considerable ingenuity to try to make their clients stand out, and thus one often sees "introducing" or "featuring" or "as." "Guest star" is mostly used (along with the even more awesome "special guest star" to indicate a one-shot appearance in a TV series.

edited: http://www.straightdope.com/columns/read/339/in-tv-and-movie-credits-what-do-star-co-star-guest-star-etc-mean