- They have not thought about what they are going to say before they arrive
- They do not understand that interviewing is a sales presentation
- They have not thoroughly reviewed the job description
Second. You have to spend a good amount of time organizing your qualifications and deciding what skills/strengths/experience are the most important to present in the interview. No, you should not develop a script to memorize, but you should have a strategy with areas that you want to highlight. Knowing what you are going to present prior to arriving will give you confidence and make the interview much easier. Make your presentation fit the job description.
Last. Make no mistake, an interview is a sales presentation. You are attempting to sell yourself to the interviewer. Interviewing is a competition between you and anyone else who may be interviewed. You need to be the best.