An interview is a two way process. The purpose from the employer's point of view is to find out about you as a person and your ability to fill the role that they have. Interviews should aim to extract relevant information from you both on a personal and professional level.
From the interviewee's perspective it should be used as a time to find out in much greater detail what the role entails, what the potential employer is like as a company i.e. the company culture that prevails and the constituent individuals that make up that company, so that any move to that employer doesn't lead to nasty surprises that should have been covered in the interview had the right questions been asked.

Interviewers are trying to find out what sort of person you are, what motivates you, what your interpersonal and communication skills are like and your technical ability to do the job. From this they will then make a judgement as to how they feel you would fit in with their existing structure and team.

Another key element that an interviewer looks at is how often you have changed jobs and why. This can give an insight into a number of relevant personal factors about you and dependent on your circumstances can give an indication of things such as how reliable you may be, whether you have a low boredom threshold, whether you are resilient, ambitious or are constantly looking for a new challenge amongst other things.

The interviewer will also be trying to ascertain how long you are likely to stay with them before moving on. Recruitment is an expensive exercise, whether an agency is used or not. There is the time and associated cost in advertising, interviewing and the administrative time and subsequent cost in producing an offer.

Once this is done there is also a "bedding in" period where the new employee is not working as efficiently as they subsequently will. This is normally during the first few weeks of a new job when someone is expected to learn new procedures and ways of doing things whilst getting to know clients and getting up to speed on ongoing instructions.
All of this is a cost both in terms of someone having to explain things and lost productivity. Therefore most employers want to make sure that they minimise their risk by hiring the right person and by utilising ever more popular psychometric profiling to confirm their conclusions reached at interview.

They will do this in a number of ways at interview, primarily by questioning, be it technical, personal or work history related. They will also observe your body language to ensure that you are not contradicting what you are saying.
Every interview will be different in the way that it is structured, the way the questions are asked and the manner the interviewer has. This may be illustrated by an interviewer seeming aggressive in their questioning or disagreeing with everything you say - this isn't necessarily an indication that the interviewer is being belligerent, it may be that they are trying to see how well you can structure an argument, how you would deal with a difficult client, how forceful you are or how you react under pressure. Although sometimes they may just be being belligerent!

There are a host of frequently asked questions at interview which could go on for pages, however following is a list of the most commonly asked questions and what the interviewer may be thinking about by asking them (although not always).

1. Why are you looking to move?
Interviewer's thoughts: What would they need to offer you in terms of prospects, work, salary, environment, clients or anything else to attract you and are realistic in your expectations. Following on from that is whether they can accommodate this.
Although it sounds obvious, be careful in the way that you speak about your current employer, even if they are the most unprofessional outfit in existence be careful how you put it - an interviewer is looking for tact and diplomacy and some form of loyalty however faint it may be! Try to find one redeeming feature.

2. What are the reasons for your previous job moves?
Interviewer's thoughts: Is this person reliable? Did they have valid reasons for moving on or not? Is it likely that this person is going to get bored here in 6 months and move on again? How much of a risk am I exposing myself to by hiring this person? Is this person ambitious? Do they have career plans? Are they structured in their approach?

3. What do you see as your biggest success in your career to date? This can be specifically project related, generally work related or from a personal achievement point of view.
Interviewer's thoughts: Is this person competent at what they do? What motivates them? How will this person best be managed? What are this person's personal values? How much responsibility does this person really have? Does what this person is telling me correlate with what is on the CV?

4. What do you see as your biggest failure? Well, there does have to be a nasty one!
Interviewer's thoughts: They are not normally interested in what it actually was - unless of course it resulted in a professional negligence case, moreover they are actually looking to see how you handle a difficult question. They are also trying to see what your thought process is and if, when put in a difficult situation, you lose the ability to think and communicate coherently. Simply saying "I haven't had one" doesn't normally work, it just demonstrates to the interviewer that you are lying. It doesn't matter how minor it is, it is useful to have something that can illustrate your ability to solve problems, create solutions, think laterally and learn. Normally this type of question is followed by "How would you do things differently now?"

5. What can you tell me about XXX?
Interviewer's thoughts: Normally this is a technically based set of questions and directly relevant to the role that you are being considered for, although it has been known to be completely irrelevant on occasion. The purpose of these questions, assuming it is the former, are to ascertain your level of technical knowledge and your ability to do the job.
Other types of questions you may be exposed to are:

Scenario based questions: You are given a scenario and asked how you would solve a problem - this can be to ascertain many things but generally, unless of a highly technical nature, are trying to find out how you react, think and interrelate with other people/clients, etc in certain circumstances.

Personality based questions: How would your best friend/worst enemy describe you? What is your greatest strength/weakness? What motivates you? What do you enjoy doing outside work? - these are really to determine how you perceive yourself and try and ascertain what sort of person you are and how that will fit in with their culture and requirements.
There are no guaranteed ways to ensure a successful interview as every potential interviewer is looking for something slightly different and has differing amounts of interviewing experience. Be communicative, however, (although avoid going on and on), be relevant and truthful and always expect something that will throw you off balance - a good interviewer will want to see how you handle something out of the ordinary or pressure as you will need to be able to cope with these things should you end up working for them.
It is always a good thing to show that you have done some research on the company, even if it was only thirty seconds around their web site. It shows a commitment to the interview and an interest, always try to have a question relating to something based on what you know about them.

Finally, as stated at the beginning, an interview is a two way process - you should have questions that you need answered, what these will be will depend on your own reasons for moving, your current environment and what you are looking for. Good neutral questions include: "What is the company structure?", "How does one progress through the company?", "What is your policy towards training and development of staff?"

Try to avoid the "When do I get a company Jag?", "What is your overtime rate?", "How many sick days am I allowed a year before being sent to the company doctor?" type questions.

However, it is important that you reassure yourself that should you be called back for a second interview or be made an offer that you are in a position to decide whether to proceed with as much certainty as possible.
It would be a shame to miss that perfect job just because you hadn't asked the right question.