There may be many reasons for his half-baked preparation for the interview. Probably it is the usual Monday with the week-end hangover. It is challenging for you to make an impression and turn the situation to your advantage. You can do it if you are a bit assertive.
Here are a few problem situations you may encounter:
The worried interviewer
The hiring manager arrives unprepared and late. He may have forgotten your resume. You can save the situation by giving the interviewer some time to settle down and even sympathize with him for having busy compulsions.
You may try to get the interview rescheduled and if you can't do it, you can help the interviewer by saying, "Is it okay if I tell you about myself and how I can relate myself to the job?" You must be polite and friendly. The way you say it speaks volumes about you.
The talkative manager: The hiring manager is a sort of man who talks on and on about the company, the job and other things and you have a little time to sell yourself.
However talkative he may be, he has to pause to breathe. Take the first opportunity to butt in and put a question and answer it yourself. The question may go like this: "Would you please say something about the work systems in the department? For instance, in my last job I prepared a work sheet telling what was due and the deadlines for different projects under way. All our projects hit the deadline.
Another tactic to bring the interview on to the track is to give the interviewer something to read, say you portfolio and then start a discussion about your skills and accomplishments. Treat him with respect all through, but do interrupt him, otherwise you lose time.
The inefficient interviewer: Here is an interviewer who doesn't know what he is up to. May be he has been brought to the interview at the last moment in place of somebody who hasn't turned up.
In such a situation you may have to don the robes of a teacher and help him conduct the interview. In a way, you have to take control of the situation but politely. Ask him about the company and the job just to break the ice. You can show him your resume and talk about your present job, your skills and accomplishments and turn the course of interview to the desired direction.
Be in control: When you come across any of these problem interviewers you must be in control of the situation. Clearly make your points you want to project and be sure they stick in the mind of the interviewer.
But at no time forget that the job search is a process and you have to continue with your network. Mind you this is not the only interview and other avenues are open to you. Keep your activity at a high level.
