In what kind of voice, emotional or not, what if make a mistake,etc.
You should do what feels right to you, and make it correspond with the spell. Emotion has a lot to do with the outcome and power of the working; if it is a spell for revenge or harm, then you should tap in to you anger or hatred of something, and focus that intensely into the working. Same goes for a love spell and such, focus feelings of infatuation or lust in to the working and viola! It makes it much more powerful.
As for if you make a mistake, it all depends again on the working. And the mistake really. Say if you are trying to influence someone, but a different person slips into your mind, the spell could effect that other person as well, be it a love spell or revenge. So obviously one must be very focused to lessen the chance of such things happening. I hope this helps!