Landing a job interview puts you tantalizingly close to actually getting hired. In many cases, though, handling the interview the wrong way can cause an otherwise qualified candidate to not get hired.
Sometimes there's nothing you can do. The interviewer may ask a question about your background that uncovers a hole in your experience or a reason you aren't a good fit. There are cases where no amount of preparation will be enough because you simply lack something that's not evident on a resume that the person doing the hiring really wants.
In most cases, people don't blow interviews for subtle reasons. Many make big mistakes that are easily avoided.
1. You show up late
Showing up late to an interview can only be forgiven if there's a natural disaster or other disturbance that keeps you from getting there. This has to be something that nobody could plan for or see coming, and even then, once you realize you're going to be late, you should call and explain why.
In all other cases, being late is a major insult. It shows a lack of respect and that you're not great at planning.
You'll want to arrive so you can walk in about five minutes before your scheduled meeting time. Do a test run the day before if you don't know the route well and to scout out where to park or other potential problems.
2. You don't know anything about the company
During an interview, you'll generally be expected to ask some questions. Those questions should show that you have done your homework and understand the company. You don't need to memorize every bit of its history, but you should be well versed in how it operates and other relevant facts.
3. You don't dress the part
An interview requires a suit unless the company specifically tells you otherwise. That can be a hassle -- and it might mean changing in a coffee shop bathroom if you're coming from a job that does not require that level of dress but it's the expectation and anything less may cause the interviewer to question your judgment.
It's possible the person doing the interviewing will tell you not to bother dressing up. This might happen if you're meeting off-site (like in a coffee shop) or if he or she knows you'd have to change. In that case, you should still wear professional attire, not your favorite T-shirt.
Take it seriously
An interview is a test. Everything you say and do may be judged. You never know when a position will come down to you and another equally qualified candidate. If that happens, you don't want to lose out because you misjudged traffic or thought a polo shirt would be dressy enough.
Treat every part of the interview seriously. Be professional and be prepared. It's fine if the conversation becomes comfortable, but you should always remember where you are and why you're there. Choose your words carefully, and let your actions show that you're the right person.