The interview has two main purposes: to find out if the candidate is suitable for the job, and to give the candidate information about the role in the organisation. Every candidate should be offered the same opportunities to give the best presentation of them, to demonstrate their suitability and to ask questions of the interviewer. Another consideration that is generally taken into consideration for potential candidates is how well they will fit with the existing team or company culture established.
You can conduct structured or unstructured interviews to assess your candidates at the interview stage. However, it is generally recommended that a structured interview works best as this will allow you to cover all the relevant information you require, accurately assess the competencies of the applicant to your job on offer and give you a consistent and equal approach to all candidates interviewed.
Unstructured interviews are very poor for recruiting the right person. These types of interviews are less likely to allow you to obtain the specific information you will need to match against your clearly defined criteria.
All interviews, whether formal or informal, need careful preparation if they are to be successful. Each candidate should leave with a sense of being treated fairly and nicely and feeling they had the opportunity to the interview their best shot. Most importantly the interview is about getting to know the candidate not the other way around.