Of course,the primary impression is rather important.And morever, you ought to show your something different from others, such as your merit, special skills and the vantage in that job.
Well,i think you should express your self-confidence to the interviewer firstly,and then prepare fully,anyhow keep smile.I think if you can do these above,you'll give a good expresson to the interviewer even if you're not empolyed.