I think it is absolutely essential to not get caught up in the character. For my oral presentation, which I got full marks for, I spoke about the need to combat poverty in the third world, without using a character at all. Admittedly, different schools have different marking schemes, but ultimately it comes down to your ability to persuade with language and this will be heavily influenced by tone of voice, body language etc.
Unfortunately no one can, without serious face to face coaching, teach you these things. However, my advice would be to know, not only your material through and through, but the order in which it is being presented [so you don't mess up].
A few other things:
1. Big words ARE good, but only when used organically.
2. Eye contact is essential.
3. CLEAR and logical argumentation is at least half of the marks.
4. Whenever you explicitly mention a dictionary definition a fairy dies.