He or she may want to use the interview to see whether you are the right fit. You can undo a favorable first impression by talking too much. During on-campus interviewing, some of my shortest interviews are with people I like who I decide early on to invite in for a second interview. Some of my longest interviews are with people intent upon wringing every last minute out of me. They keep pitching even as the next interviewee knocks on the door at the appointed time.
Employers avoid setting themselves up to have to counsel new hires about their appearance.
So if you underestimate the level of formality that an interviewer expects, expect to be rejected. Dress for job interviews like you would dress if you were going to conduct a jury trial. That's more formal than is probably necessary on the job, but it's safe-unlike underestimating the interview dress code. I'm lax about how applicants dress, yours would be an underestimate. In an interview you need to be a real person, complete with a sense of humor.