Well, I’ll answer this from past experience.
Once you have a group of prosepective actors, invite them all to meet a certain place. Have a little waiting room, and have a table that has little excerpts from the script with the character’s name on them, so that they can read over the lines that they will be auditioning.
Then, invite each one of them into the next room. Ask them to tell you a bit about themselves; get to know them and get a feel for their personality. Then, ask them to please read the lines that you provided. If you have a camcorder or one of those mini-tape recorders, I suggest you record them speaking their lines. After the first time they read, perhaps make some suggestions and have them read it again.
Then, thank them for coming and usher in the next actor.