In order to avoid any frustration, please ensure that the paper that you bring them is sufficiently looked over for spelling, structure, etc. They are only human, and can do so much. So, as a rule of thumb, ensure that you come prepared -- ready to discuss the thesis of your paper. They will be always happy to help any walk-ins (that is those who come to the Center without prior appointment). Nevertheless, it would be advisable for one; you can either make it online -- here: Main appointment page, or directly outside the schedule's board outside the Writing Center room.