There are two things to it: content to speak and the fluency with the language.
For the contents, as mentioned above, just find the interview experiences thread and go through them... you'd get a fair picture of what you can expect. Rest is about the being cool etc, which is entirely in your hand.
Abt the fluency, I guess there is no easy way out. Speak to as many people as possible... make use of co-workers if you are employed, read a lot of good books etc.
All said, Im inclined to believe that content would take you miles but fluency just adds the final polish. ATB.