The interview is to ascertain whether the applicant should be granted a visa and the questions will range from simple fact-checking to personal questions about the reason for your trip. If there is any doubt about whether replies are truthful or not, the visa application may be turned down. This is to ensure you are a not seeking to stay illegally in the country in question, and you will not enter into any unlawful activities once there. For the honest and genuine traveller this should not be an issue. If you answer truthfully and completely there is no reason why your application shouldn't be successful.