Everything Bhustighre and Sanju Baba said applies in here!!! Plus, do not worry at all. These are not formal interviews. Professors walk with you around the department, introduce you to other professors and other graduate students. May be, he/she might take you to lunch/dinner. Its nothing like job interview. One more thing, when they pay you for your visit, its certain that they will hire you as a teaching/research assistant. So, no problem at all....Have fun and yeah, like Sanju Baba said, try to be formal almost all the time. Good Luck.