In theater (or, if you prefer, theatre) callbacks are that wonderfully indeterminate stage between going to the audition and possibly actually getting cast.

Callbacks are rarely like another audition. In a typical audition, you perform one or two prepared monologues and possibly a musical number or two; in some cases, you do a cold reading of the play, maybe even with somebody. In a callback, you are often thrown a script, a scene partner (or eight) and told to go to it. Sometimes, you get a few minutes to rehearse. The director usually plays a sort of round robin, shuffling scene partners like a bridge deck.
Group warm-ups are often a feature of callbacks; I think it's a method to guage how enthusiastic and willing the potential cast members are. Also included may be: comedy improv, script-related improv, miming, dance, movement, storytelling, and even singing or musical improv. It can get wacky.

There may be multiple rounds of callbacks; for big-time TV shows, nine or ten rounds of callbacks is not unheard-of. Callbacks are often fun, but, they can also be hella nerve-wracking.



Now, this is just my personal take on callbacks, so I could be wrong (any directors out there feel the need to correct me?). If you've only got one round of callbacks (e.g., if you got relatively few people for auditions), callbacks are often more confirmation than huge decision. It's just a method of making sure that the people you're casting can take it and making sure that those ones that showed promise but just didn't fit weren't having an off day (they can also serve as handy back-ups if the prima donna you had in mind turns out to be a total bitch). The bad part of this is that when it happens, you usually know. And when you know, you know which category you belong in; it's weird to be able to pick out the cast list before it gets posted (especially when you're not on it).