The IELTS Speaking module is scored to a large extent on your ability to maintain a smooth conversation for an extended period of time.
Simple, just show that you understood the original question, then expand your answer using a topic that you know very well.
For example, if you are asked to talk about your favorite person, but you really want to talk about cats, you can name a uncle as the favorite person, you can say that he is a cat lover and now you can talk about cats to your heart's content, just remember to refer back to your uncle from time to time.