From the book How to Prepare for an Employment Interview.
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[Chapter 3: continued from here]...How can you help them solve their problems and achieve their goals?
What do you bring that's of value?
Why is this better than what the next person has to offer?
These are the questions you want to answer. No matter what the interviewer actually asks you, if you're not REALLY answering one of these questions, or providing evidence to support your answer, you're boring the interviewer and wasting your time.
Almost all of the interview will be spent talking about you, but the interview isn't about you at all. It's about the employer and his problems and what you can do to solve them that others can't do as well or better.
Tap into the employer's aspirations ... their goals and desires
If you don't know the answers to these questions-or at least have a pretty good idea-then your interview is reduced to spinning a roulette wheel. Sometimes, by some fluke of luck, your number will come up. Sometimes you'll happen to say what they want to hear. But not usually.
You can improve those odds with preparation. Preparing for the interview in advance is the only way you're going to go in knowing how to answer the fundamental question-"What do I gain by hiring you?"
The only reason any organization hires anyone is because there is something that they aspire to achieve-an objective or goal-that they can't get, or don't think they can get, with their current staff. The objective may be a heartfelt aspiration that the employer deeply longs for, or it may be something more mundane. But one way or another, every hiring situation indicates an objective that the employer wants help to achieve or a problem they need help to solve.
You want to tap into that desire. To paint a picture for the employer of how they will achieve their objectives ... with your assistance. To let them know how they will benefit from bringing you on board and giving them the evidence they need to believe you can help them achieve their goals.