It's said that a good improviser is one who possesses numerous cliches and artistic licence at hand for all cases. I had a chat with a teen last night. She told me that she'd never known what she would start with to give a talk. And it doesn't mean that she's uneducated nor does it say she's word-bound.
Another case, today I had a chat with a young woman, who is, mind, a software programmer, from India. She's taken her first speaking IELTS test today. We had a lot of chats before in which she showed herself fluent and resourceful enough when discussing various themes. But this time in the exam she got lost at speaking part 3 for her nerves were so extremely strained that she wasn’t able to focus on what the interviewer asked her. Examples of this kind abound.
In my opinion It is not worth learning all possible topics by heart. A handful of individuals possess impeccable memory. One of possible solutions is to start a talk with a general reference to the theme and common problems.