Before you begin a Ph.D., it's useful to know what knowledge and skills you'll be expected to already have. Find out from potential advisors so you can brush up before you start.
Your potential supervisor may already have a research project in mind that he'll expect you to undertake. Ask for specifics. You're entitled to know what research you'll be doing before committing yourself. You and your advisor must agree on who's in control of the direction of research. Some advisors prefer to dictate the terms of your research; others are happy to let you have more control if that's what you want. It's important to work with an advisor who suits you in this respect. It's also difficult to change established patterns later on, so ask about this at your interview.
Find out if potential Ph.D. advisors get actively involved in research themselves, or whether they tend to leave it to their research students and concentrate on supervision. Academics can become too busy with consultancy, teaching or committee work to be very hands-on. If this is the case, you may not see these people very often, and issues around recognition may also become important if you're doing a lot of research for them. Ask about how decisions on paper co-authorship are made, for example. Finding out who gets credited first and why, for instance, can be useful. Is it by contribution or simply by alphabet or hierarchy? These issues might not seem important now but may become so later on.
These issues can become vitally important to you during a Ph.D. At vital stages, a lack of time commitment and too little, or ineffectual, feedback from an advisor can be devastating to your progress. Ask your potential supervisor how often she'll be able to meet with you, how she prefers to communicate and how detailed, or otherwise, her feedback will be. Ask how many other students she supervises; this can affect how much time she can commit to you.
Find out what your responsibilities will be in terms of writing proposals and presenting to research sponsors. Also ask whether he expects you to prioritize research or coursework, and what length and standard completed theses and other coursework should be. Specifics can help you to differentiate and decide between alternative potential advisors and Ph.D.s.
Finding out more about the advisor's personality and your work environment also helps. Find out how the supervisor deals with conflict, whether she socializes with post-graduate students and how supportive she is of a student's future careers and progress -- for instance, whether she stays in contact. Ask how closely her students work together, and ask about their work habits. A laboratory where students tend to arrive late and work through the night will not suit an early bird.