Unless you work entirely alone, the day will arrive when you will be in a position to select a partner of sorts. Whether seeking a band member, an employee, or someone with whom to share studio space, the decision is crucial because you will commit time and energy to working with that person and, to one degree or another, you will tie your fortunes to his or hers.
The process by which you choose your cohorts has everything to do with how well things turn out in the long run. Good information and solid techniques can be garnered from the numerous books, seminars, and courses about job interview methods, but auditioning or interviewing for a band or some other creative enterprise differs in important ways from traditional job interviewing.
In any situation, you evaluate candidates' qualification for the position, be it their ability to build a compelling sci-fi ambience or their repertoire of Chicago blues riffs. The standard tools, demos, and résumés provide a good start toward determining those qualities.
But those are just the basic prerequisites that get a candidate in your front door. I think the most important criterion is how the person fits into the situation and works with everyone else involved a much trickier thing to determine, as it involves that elusive quality: chemistry. (For this discussion, I will forgo discussion of the legalities in interviewing and assume you are abiding by all applicable laws.)
So what's so different about auditioning or interviewing for an artistic enterprise? In a creative effort of any quality, emotion is necessarily involved. Of course, you want to be as professional as possible, and part of that is managing how emotional content is manifested. The emotion is present nonetheless, and trying to pretend it isn't leaves you vulnerable to later finding out the hard way how much it matters. If people trying out for a band have no emotional involvement, they are darned unlikely to be expressive players, something generally desirable even in straight-up session situations. Those who can't control their emotions will be counterproductive at best.
Creative and personal orientation that is, attitude are also very important. Those are largely subjective evaluations that the business world couches in terminology such as team player and degree of motivation to make them seem less so. It still comes down to a question of who the person is.
I've been involved in quite a few auditions and interviews, and the techniques I have learned for uncovering that sort of information frequently amount to asking questions whose answers lie at more than one level: the direct answer to the question itself and the candidate's decisions in responding.
When interviewing guitarists for one band, I did phone screenings before devoting time to personal meetings. As soon as the candidate identified him- or herself as responding to my ad seeking a guitarist, I asked the person to tell me about him- or herself as a player, before I even said a word describing the band.
Some answered with a list of their equipment, others told me of their influences, and yet others described their experience. In each case, I immediately learned something important just by how he or she chose to respond to that deliberately broad question.
In another interview situation, I asked what made the person lose control and what happened then. If the person wasn't immediately forthcoming, I pressed harder to get the answer. Why? To see how the candidate responded when pushed, because the job would sometimes entail high-pressure situations.
One must always take care in auditions and interviews to maintain an atmosphere of respect, professionalism, and courtesy (as well as legality), but creative jobs call for creative tactics when making personnel choices. Try some inventive techniques, and I guarantee you'll be amazed at the insights you'll gain.