Normally the interviewer has far more practice at employment interviewing than the applicant. Preparation and practice can reverse the normal advantage. At some point in most interviews, the interviewer asks something completely open-ended where the candidate is given free rein to describe him or herself. If your answers don't help the interviewer figure out what an asset you'd be, you've failed in your sales job. Be positive and describe yourself in terms of successes and goals as they pertain to the job.
Nobody knowingly hires a negative person.
Describe your skills and talents with People Information Tools. Then ask if there's an area about which the interviewer would like more information. Use specific examples of behavior that would benefit the interviewing organization, e.g., problem-solving, nurturing customer relations, saving money, etc. Use illustrations, examples, success stories, anecdotes. You need to be careful not to project attitudes that say you think other people are for the most part lazy or dishonest or just plain stupid.