An interviewer has a list of prepared questions to ask each candidate. However, the interviewer also finds it useful to ask some unplanned questions when the candidate mentions something interesting or when it seems appropriate to gather more information. This describes a(n) interview.

open-ended interview

Explanation:

In an open ended interview, an interviewer is open to ask new question to the participants based on their responses in order to discover new interesting facts which can be useful for the study/research under which this interview is being conducted.  

In a closed end interview a participant is a usually allowed to answer is one word either yes or no, true or false or some sort of rating inputs such as good , better , best etc.