Do not hesitate to ask questions
The worst thing you can do during a job interview is to keep quiet and not ask questions that could be cropping up in your head. Do not interrupt the interviewer as it sounds rude to ask your questions when the person sitting in front of you is expecting you to answer their questions. The best way to do it is, make a list of questions that form in your head during the interview and ask them one by one at the end of the interview. There could be many things about the company which you do not agree with, so things should be clarified.
No matter how bad the job offer is, do not sound rude. Being polite will help you share your thoughts with the interviewer in a more effective manner. For instance, if the interviewer offers you a salary way less than you expect, do not start jumping or being aggressive towards the interviewer.
Be as honest and upfront as you can during the interview. If the job they are offering is way beyond your capabilities and expertise, tell the interviewer about your core strengths and expertise. Tell them that you do not fit well, but you can be a great asset for the prospective employer if you are given a chance to do what you are an expert in.
Ask about other positions that may suit you
It is not that unusual for the interviewer to offer you a job that you find difficult. In that case, do not just shake hands and walk out the door—ask the interview about other positions available. If the company is recruiting people for multiple positions, you might get a chance to show to the interviewer that you are capable of working in other positions, despite the fact that you are suitable for the actual position you had initially applied for.