Here are some things you should bear in mind for those on both sides of the interview process. For example, if they like to go to bed early and you like to party into the night, you may wish to ask yourself if this will cause problems in the future. Think about your own habits. If they strike you as very neat or very messy, ask yourself how this will fit in with you. Try to get a feel for the person.
Personality is very important.
Make any ground rules clear. Have somebody else present at the interview. This is a good idea for both personal safety reasons and also to get a second opinion. Have a series of questions planned and allow enough time for each of the applicants. Leaving a small gap between interviews will allow you to regroup and prepare for the next interview and will also avoid applicants overlapping.