Most actors attend an audition with one goal in mind: to get a callback. A callback means that the director saw something in your audition that he liked, and is considering casting you in a role. At a callback audition, actors generally sing songs from the show, read scenes with other actors who have been called back, and are sometimes asked to dance.
Speak with the director. In community theater, directors will usually notify the people they wish to see again a day or two before the callback audition. When you receive the call, be professional. Even if you're excited beyond belief, save the screaming and jumping around for your friends and family---after you get off the phone. Be sure to thank the director for considering you.
Make sure you understand all of the details about the callback before hanging up the phone. Know where the callbacks are being held, on what date and at what time. The director should tell you which role you have been called back for, and what you will be expected to sing. You should also find out if any dancing will be required at the callback audition.
Research the show if you're not familiar with it. Become especially familiar with the role you've been called back for. You can find detailed synopses of most plays and musicals on the Internet. If you have access to iTunes or another music service, consider downloading a song or two so that you're familiar with the musical style of the show.
Pack for the audition. Bring extra copies of your headshot and resume (the director should already have both, but it never hurts to be prepared). Bring dance shoes and appropriate dance attire if a dance audition is required.
Warm up in the car on your way to the audition. Arrive early and sign in. Turn off your cell phone, or at least set it to vibrate.
Think about your character before singing or reading. If you've researched the show and the character, this shouldn't be difficult. If not, familiarize yourself with the song and reading material as quickly as possible to get a feel for the role. Be in character as much as possible when singing, reading and dancing.
Thank the directors and the accompanist before leaving.
If possible, plan to wear the same outfit you wore to the initial audition. Style your hair the same way, too. Directors will sometimes have an easier time remembering you if you look the same as you did the first time they saw you. You usually won't need to bring sheet music to a callback, as the songs will all be from the show and the accompanist will have the score.
Don't talk, eat or be distracting when other people at the callback are performing. Directors do pay attention, even when you're not performing. Be respectful of others at all times. Be careful when researching a show that also has a movie version, and make sure you know what to expect from the stage version. For example, in "The Sound of Music" movie with Julie Andrews, neither Max Dettweiler or Baroness Elsa Schraeder do any singing. In the stage version, however, they share two duets. Anyone called back for either of those characters should know that they have to sing and be familiar with those two songs.