This is a question that came to us from a member of our Private Facebook Group (Feel free to join). You can ask questions about the IELTS exam and get answers from IELTS examiners, the only people that know what you need to do in the exam.
No. What gets you into the high scores is being able to discuss a range of topics relatively fluently and having a sufficient range of vocabulary that allows you to present your ideas in a clear way.
A lot of people learn English from word lists and the “Don’t say ‘very’…:” posts that you see on social media. They are nonsense. Native speakers have to do IELTS for immigration purposes and they don’t even prepare but they still get Band 9 (99% of the time). They don’t come in with the idea that they are going to avoid using ‘very’ or need to use complex vocabulary. They come in, sit down, have a chat for 14 minutes, answer the questions they are asked and leave. They can talk about a wide range of topics fluently.
The range of vocabulary they use is wide enough to talk about anything that IELTS throws at them and they probably say ‘very’ 20 times (it is a very high frequency word, oh I said it!!).
The speaking test is a test of your English, nothing more and having a great vocabulary won’t help you very much if you can use a/an correctly, or don’t have any ideas to support your opinions.
So, in short, in answer to your question, no, you don’t have to use fancy vocabulary in IELTS speaking. Be yourself and this will get you higher marks.
If you want to do an IELTS Speaking Exam with an IELTS examiner online, you can order one here. You get a full report (see the example here) and advice on what you need to do to get higher marks.