Let me just offer a caution.  The director(s) saw something in your audition that they liked and think they could use in the show.  You probably do not know what it was but be careful that you don't lose that in the call back.  My suggestion is to not do anything to change what you did at the audition unless the director gave you specific instructions on any changes. 

If you haven't read the script before the audition, read it before the call back so you have an idea of what is happening to the characters you are being considered for. 

The problems in changing how you approach the charater you are auditioning for is that you might move away from what the director is looking for.  I have been involved in call backs where a person who was charming and sweet in the audition was an angry young woman in call backs.  The director (and the rest of us at the call back) wondered what happened to the first young lady.  In another case, a woman came to call backs and was so over the top that the director asked her what she was doing and she said that it was how she interpreted the character she was called back for. 

I audition a lot of people and the cold reading is just part of the selection process.  Does this seem like a person I want to spend the next eight to ten weeks working with?  How do they take direction?  How does their voice project?  How do they carry themselves before and after the reading?  I can't tell you how often people change (for the worse, usually) between the audition and the first read through.

Just my two cents worth.