Pointing to a black case he'd carried into the interviewer's office, applicant stated if he were not hired, the bomb would go off. Disbelieving, the interviewer began to state why applicant would never be hired and that he was going to call the police. The applicant then reached down to the case, flipped a switch, and ran. No one was injured, but the interviewer did have to get a new desk. When in doubt, dress somewhat more formally than you think would be expected for the job.
First impressions are very important.
In addition to what you are saying, your non-verbal behavior is very important in an interview. Practice with a friend or in front of a mirror to assess these factors. Ask yourself what can you contribute to the company. Failure to ask questions - Employers will assess your interest in the company and the position by the types of questions you ask. Proper research prior to the interview will help ensure that the questions you ask are intelligent ones.