Hi,
My husband works for WJ, and I'm kind of familiar with how it works. Since no one else has responded, I'll tell you the things I know.
A lot depends on the position you are applying for. If it is a "skilled" position (such that you took schooling or training previously) then a lot will depend on your experience and resume. They will be judging your personality as well, and are fond of using behavioral interviews. Run through your answers to typical behavioral interview questions before the interview.
If it is a "trained" position (where they will train you, such as a flight attendant or CSA), then most of what they will be looking at is your personality and how you interact with others. I know for flight attendant positions they often do group interviews where about 10 people are interviewed at the same time. The first most important thing is not to be late for the interview! I have heard that they will lock the door and you will not even be interviewed if you are late. (this would also probably count against you in the future) The key for these positions is to let your personality out. Do not try to be someone you're not, but if you have a great personality, don't be afraid to show it. Since they will train you for the specific job, they will want to know about the things they can't train, such as how you are at customer service, and how you deal with stressful situations. Most of all - be yourself.
I hope that helps a little bit. Good luck!
PS. It is a great company to work for. Depending on the position you're in, you may not be making a lot of money, but if you take full advantage of the employee stock plan you can increase your salary/wages by 20%. You also get the benefit of airline employee travel deals, so if you've always wanted to see the world it can be a great opportunity that way.