Always research the company that you are going to be working for.  Go onto their website and see what they do, if they are national or international, what services they offer, etc.

Always make sure that you make a good first impression as this is the only time that you will be able to convince the interviewer that you are more capable of doing the job than any other candidate, as a job is usually offered based on the assessment of your performance during the interview.  

What then, are the criteria for selection in this process?

It is always best to select key tasks from your own experiences which demonstrate that you can do the job, and will do it better than any other candidates.  This is done by asking the right questions, knowing the problems faced in that position, and even offering the solutions to such problems. 

Remember an interviewer is not trying to trick or trap you, nor are they going to penalise or find fault with you.  In fact, they are most relieved if you can convince them that you are the right person for the job and the position will not be vacant anymore.

Here are some of the questions that might be asked of you in your interview: 

Avoid using "trial and error" in job interviews, by making mistakes in front of your prospect employer. Tactful answers to the above questions will impress the interviewer and most importantly of all, you will stand out among other candidates to get the job offer.