As one of my university professors said before exams: "I know which parts you don't know." Meaning that if he wanted someone to fail, he could easily arrange that. This was his way of motivating students to attend the lectures.
There's really no way to prepare for the full set of eventualities, and unless you do something spectacular (chances of which are slim in a high-stress situation), their opinion of you is formed in the first few minutes of the interview anyway.