While it may be impossible to literally prepare for any and all interview questions or scenarios, you should do your best based on available information and at least be mentally prepared for the rest.
With an upcoming interview, your first priority is to do your homework. Thoroughly review the job description, any available information about the firm or company posted on the web, and bios on the people key to the job and interview process. You should also review your own resume and any supporting material you provided so you are ready to address specific questions about the material you provided. These are things within your preparedness control and the employer often is aware of it, which means your interview process could end quickly if you are not up to speed on easily accessed information.
The trickier part of interviewing is preparing for questions or scenarios for which you have no or limited access to information or control. This might be a case study posed to you by an interviewer regarding something you may or may not have experienced in the past. It might be dealing with a quirky interviewer who keeps going off-point during the interview. While you may not be able to prepare a well thought out response for everything, you can prepare for how you respond in terms of your demeanor. The key is to anticipate difficult questions or scenarios mentally so you are not completely caught off-guard when one comes your way. Often times, the interviewer is not as interested in the answer itself, but how you process information and your composure in responding.