What is the difference between a gueststarring and co-starring in a TV shows credits?

I think a co-star is in every episode while a guest star only appears once in a while but has a major role. For example Sarah Michelle Gellar guest staring in Angel.

In show business, a guest appearance is the participation of an outsider performer in which the participation of said performer does not belong to the regular cast and crew.

A co-star can be defined in the following ways:

- a performer, esp. an actor or actress, who shares star billing with another.
– a performer whose status is slightly below that of a star.
– to present (two or more actors) as having equal billing or prominence.

I would add that a guest star is likely a person who is already famous. It gets people to watch when they can advertise that Jerry Seinfeld is in the next episode of 30 Rock e.g. On the other hand, if it’s, for example, a crime show and they have people involved in the case (criminals, victims) who are only on for one episode but play a big part, I don’t think they’re “guest stars,” just ensemble character actors.

Billing is negotiable. Your agent would talk with the casting director about your billing, but generally, there’s featured (you have two – five lines), co-star (you’re in a whole scene or two) and guest star (you’re very famous and/or your part is integral to the episode and you have a lot of scenes).

The best case scenario for the guest actor is that she gets a separate card at the opening credits. That means her name is on the screen by itself at the beginning of the show after the regular cast is listed. Next would be a shared card (her name shares a screen with other performers) at the beginning of the show, and on down to being put in the end credits where only her mom will look for it. Extras, of course, get no billing whatsoever.