One of the first ways are affected by candidates is by the way they seem to handle themselves from the outset. Although, ultimately an examiner is judging a candidate's whole performance. The initial speaking responses to the first questions are already forming in the examiner's mind a kind of theory about the candidate's band level. One of those cues is the degree to which a candidate struggles when responding. I will talk more about the struggle signs but for now I will confine myself to the actual literal response.
In general, the more information a candidate is giving, the more energy is being used trying to think of new vocabulary, structures and connections. Trying to give too much information right at the start of a speaking interview can be difficult and struggle may show up very quickly and stress may result in mistakes that a candidate might not normally make. Therefore, a strategy to extend the length of answers without giving more information is simply by giving fuller grammatical answers.
A short fragment response put more pressure on the candidate to extend because the candidate feels pressured to carry on and give more information. Look at the two responses
Q. Where do you study?
A1 In college
A2 I study at a college
One trick is to try and follow the grammatical structure of the question into your answer
Q How long have you been studying?
A I have been studying for…….
Notice the end part of the sentence (your adverbial clause) often becomes your Starter clause
Q What will you do in the future?
A In the future I will probably .....
However be careful not to make your responses sentence answers artificially follow the question grammar - this gives the examiner the feeling that your responses are unnatural and formulaic, which can work against you if you struggle later on to speak more spontaneously with deeper questions that are hard to anticipate.
Q Do you think that it is always a good thing to help friends when they are financially in trouble
A1 Yes I think it is always a good thing to help friends when they are financially in trouble because.......
A2 Yes I think it is always a good thing because...
Here it might be more natural to just say A2 because it sounds more natural and comfortable and gives you an easier structure
So…. Use full sentence answers but keep them conversational.