An interview usually takes 1/2 hour, although some companies do several interviews or tests (e.g. one professional, one personal, etc.). Interview structure varies, but the atmosphere is almost always formal (even if it is friendly). You should be slightly reserved unless you see the employer adopt a more relaxed attitude. The content of the interviews is usually job-oriented, but there is an increasing tendency to cover personality, skills and attitude, intelligence and general behaviour. You should always try to sell yourself explaining your motivation and why they should choose you, without of course being too pushy. Often the CV is referred to during the interview, and candidates are almost always asked why they left their previous job and to name their five best and worst characteristics.
You can ask anything about the job, but you should not start the interview by asking about the salary - this topic is normally raised by the employer. You should, however, be prepared to discuss your salary expectations, in which case it is always better to give a range than an exact figure. Nowadays the applicant is the one who has to state a price for his/her work as an office employee.