I've interviewed lots of prospective graphic designers - and I've learned a great deal about what to expect and what to look for in a graphic design interview. Here are some of the things which I find to be a complete turn off - some may seem obvious 'no-nos', but they happen again and again.
* Don't say that your grand plan is to learn as much as possible from your first job and then strike out on your own as a freelancer or to set up your own studio. Your ambition to steal your prospective boss's secrets and then set up in direct competition won't make you any friends.
* Don't appear desperate for the job - if you say that you've been to loads of interviews with no luck (and now you have to get a job or you don't know what you'll do) you won't get the job.
* Don't whisper - speak clearly. Don't fidget or look away while the interviewer is talking.
* Don't moan about how badly you were treated in your last job, or how much you hated your boss. If you do, your prospective employer may think you'll talk the same way about him or her behind their back!
* Don't produce vast amounts of irrelevant work from your college portfolio - just produce designs which would be at home in a commercial arena (adverts, stationery, brochures etc). My experience of college work has been mixed. Usually it comes over as somewhat (and I certainly don't mean to cause offence by saying this)... self-indulgent... and decidedly uncommercial. Many's the time that I would have got more out of seeing a couple of logo and stationery designs than I did out of looking through three years of abstract batik and photos of bicycles made of cheese.
* Don't give the impression that you're just there for the money. The employer won't care nearly as much about what you can get from the job as they will be concerned about what you can give. Employing a new graphic designer has to be a profitable decision. If your contribution is unprofitable, your job won't last long.