At the first audition, it's easy to not want it so badly. The carrot at the end of the stick seems very far away. You can go in, relax, and have fun.
But as you get closer to booking the job, the stakes seem higher, the pressure mounts. You walk into the waiting room and you see your competition, sometimes even recognize their faces or their names on the sign-in list. Then you walk into the audition room and instead being with one casting director (who you might have already have a great relationship with), you're in a room that may be filled with producers, writers, and god knows who else. The tendency is to allow the callback to become a more stressful and difficult situation. But the opposite has to be true. The closer you get to booking the job it has to appear that it's easier and easier for you. That way they know that when you eventually do get the job it'll be a piece of cake...or as easy as pie!
How do you achieve this enlightened state of mind?
Know that a callback means they like you, they really really like you. Don't think the casting director made a mistake or is calling you back in the hopes that you can pull it together with a bit more time. He thinks you have a great chance of booking the job and wants to show you off to his bosses (the producers and director). If that isn't a great vote of confidence, I don't know what is!
You've had a bit more time with the material, so you know it better and perhaps even got a bit of helpful direction from the CD. However, resist the temptation to drastically change your performance at the callback. The casting director will be very miffed if she has to explain to the producers (after you've left the room) that what they saw was nothing like what she saw at the pre-read. Wear what you wore to the first audition, or something very much like it, unless you got a note to wear something different.
You have much less competition once you've reached the callback. Maybe you've now got a 1 in 6 chance of getting the job rather than a 1 in 30! Look at your competition and, instead of feeling intimidated, decide who's going to be second choice to your first choice. Nobody has what you have. Nobody can play the role the way that you're going to play it. Play the audition as if the role is already yours.
And don't forget that even though you've now had more time with the material it has to seem like it's the very first time you're saying those words. Don't let it get stale. Use the excitement of the callback situation to keep it fresh.