I always find it impressive when candidates ask a couple of questions at the end of an interview, as this demonstrates an interest in the firm and an eagerness to find out more. It is fine to write a few questions down on a piece of card and keep this in your pocket. Questions should be thought through before the interview and remember not to ask questions on areas that have clearly already been discussed during the interview. It is always good to ask about the strategy of the firm and where the firm may be by 2009. Another good question is asking the partner why they joined the firm. It is also fine to ask more basic questions but remember most firms give you an opportunity to meet trainees after the interview so you can always ask questions then more informally if you prefer.