<p>For a school like Duke where the interview is optional it is okay not to do an interview and submit the common app after Dec 10. However, once you have been offered an interview you honestly MUST accept the invitation if you have any interest in going to the school. Interviewers and schools will not think highly of someone who rejects their opportunity to depict themselves as a prospective student. I do not know how much weight the interview really holds in the process. All I know is that you should take the interview if you want to get accepted.</p>