You should speak as you normally do. If you try and do things like
1. Putting on an accent
2. Attempting to use words that you don't really know the meaning of (many people try and fail at this)
3. Idioms that you aren't familiar with
etc, you will end up hurting your score rather than helping. The speaking test is designed to see whether you can communicate effectively. If you don't have a wide vocabulary/aren't familiar with idioms, and you try and squeeze them in, it will be extremely obvious to the examiner.
Keep it simple. Effective communication does not require big words or idioms, though naturally those can make communication more effective, when used correctly and appropriately.