If your goal is Band 6, you don’t have to speak complex English – just don’t make mistakes in simple English.
Don’t experiment with new words during your interview – it is not the time. Use only words and phrases you are sure about. For example, if you were reading a newspaper yesterday and saw a new word, but you’re not sure what it means or when people use it – forget about using it in IELTS.
No one will judge you on your ideas, only on your English. If there is a movie you can’t describe (your favorite) and there’s a movie can describe (that you have recently seen) – choose the second one. They are not interested in the truth, only in your English.
You have to think about what and HOW you are going to say.
Expose yourself to as much English as you can. Not just to learn new words and phrases, but to also practice until you master them.
If you make mistake – that’s bad, if you correct yourself – it’s not that bad. Examiner doesn’t count your mistakes, and it makes a good impression when you show that mistake you have made was not intentional.
If you say “economic” instead of “economy” and make more mistakes like that – its band 5. If you make a couple of such mistakes – it could be band 6. If examiner can’t understand you – it’s band 4.
Band 7 people should show they can use a little of special vocabulary. Explain ideas very well, very clearly, and use not just simple words, but more special words, like “economic environment”, “favorable conditions” etc.