One of the gravest mistakes you can make is going to the test center not knowing what is coming. Before your test, make sure to thoroughly acquaint yourself with the test format and pattern by taking some practice tests, so that there are no nasty surprises on the exam day.
It’s more than enough if you talk as you normally would to a friend or colleague. That said, it is important to make good eye contact with the interviewer, smile every now and then, and talk politely and courteously.
The objective of the IELTS is to test your English language fluency, so it is necessary that you answer at least at some length and do not simply give monosyllabic answers.
Remember, the interview questions are usually straightforward and simple, and don’t need more than a 3-4 sentence long answer.
If there’s a word or phrase in the question you don’t understand, it is perfectly OK to ask the interviewer to clarify it, but choosing to give no answer will be seriously detrimental to your overall score.
It is usually very easy to tell when a candidate has memorized his answers, and doing so is likely to reflect poorly on your scores since the interviewers are looking for spontaneity and ability to respond correctly and quickly.
A native-like fluency is not expected of you, but it is important regardless to only use words whose pronunciations you are confident about; otherwise, choose a synonym you can actually pronounce.
One of the things the IELTS seeks to examine is the breadth of your vocabulary, and using the same word over and over again will not win you any favours.