OK, a sweeping title. There are many, many reasons for people failing at interviews and it's only working with individuals that one can get to the bottom of a particular case. That's why I don't believe in "career success" books. That said, there are some themes in all of this.
Although I've been coaching people for job interviews for well over a decade now, I'm still regularly surprised at how intelligent and highly competent people waste their opportunity to impress at interviews. It's not just junior people, or people who have worked for the same organisation for many years and are therefore unfamiliar with the interview process. I see real heavyweights, senior people from big organisations, who really struggle to make their mark at interviews.
What's so crazy about this is that it's really easy to standout from the crowd. Leave aside all the hygiene factors like getting there on time, making sure your clothes are straight and creating a good first impression. All those things are important but unlikely to set you apart from anyone, except in the wrong direction if you get them wrong!
I'm talking about preparation. The thing is, most people think they know what they are going to say because they feel on top of their careers. They know they are competent enough and believe that they'll be able to handle any questions that come their way.
Big mistake.
Here are just three common problems.
Preparing in your head is not the same as practicing out loud. People think they know what they are going to say, but it never comes out the way they imagine it. They tend to think while they are talking, and this leads to waffling and losing the thread. Often, by the time they finally finish talking they can't remember what the question was, and the interviewer has no idea what they are talking about.
Second, even 'though people only think about questions looking at competence, they often don't know how to give answers to even those questions. Answers that are clearly structured with examples of past performance. Answers that demonstrate how they made a difference.
Third, there are nearly always predictable questions that most people struggle to answer. Examples include "Tell me about your main weakness", "Tell me about yourself", "Where do you see yourself in 5 years from now?", "What would your last boss say about you?", familiar questions? Of course, they come up all the time. And yet most people don't know how to answer them well. If you do answer them well you'll score massively in an interview. What's more, these are not difficult questions. In fact I would say no interview question is difficult if you are prepared for them and if you understand why they are being asked.
Let's look at that in more detail. There are, essentially, 3 types of interview question and almost all questions will fall into one or more of these categories. If you can work out which category a question falls into you are already halfway to a good answer.
a) Competence: can you do the job.
b) Motivation: do you want the job for a reasonable length of time
c) self-awareness.
The "weaknesses" question, for example, is about self-awareness, not, as many people think, competence. Knowing that, you can see the importance of getting it right. Instead of thinking the interviewer is trying to find out if you are incapable of doing the job, you can now see how the question is really aimed at how you learn from, and avoid repeating, mistakes.
That's just one simple example, but it all comes down to preparation. If you've done your research you can usually predict quite accurately what they are going to ask, and if you know what they are going to ask, you can prepare the answers you are going to want to give.
One last point for now. The real trick to succeeding at interview, perhaps more important than the questions themselves, is to create and build rapport. That's how the fourth element of what they are looking for is established. The fourth element is "will you fit into our team / organisation?". You can be the most competent, the most motivated, and the most self-aware person in the job market, but if you can't create a relationship with the other people in the room the chances are you'll not be asked back.