Actually there is no such written rules. That depends on the question asked. If the question and the marks set for it requires some descriptive or analytical answer, you need to go that way. The only thing to remember, you should not repeat the same point in an answer. Also, avoid adding those details in case you don’t remember the events of the story properly and afraid of mentioning something wrong.

In short, read the question carefully twice or thrice and try to understand what the questioner actually wants you to write and then plan your answer in a few seconds and start writing the answer.