One of the primary skills when doing voice over is knowing which words in a script to emphasize. (Or not emphasize).Of course, you'll have many choices. If you're being directed while in the booth, the director/producer may tell you exactly which words to emphasize.
Let's take this very simple sentence and see what the variables are for emphasis.
"Did you walk the dog today?"
If you emphasize the word "you" that would imply you're asking whether a said individual walked the dog as opposed to some other individual.
If you emphasize the word "dog" that would imply you're asking whether a particular person walked the dog as opposed to some other animal.
If you emphasize the word "today" that would imply you're asking whether the dog was walked today as opposed to yesterday.
You can click here to hear the audio for the sentence.
So, as you can see, a sentence meaning can be drastically changed with emphasis. Knowing which words to emphasize before even turning to the microphone is key.
There are many ways to emphasize a word when voicing. Many get slightly louder when they come to the key word. But you can also emphasize a word by getting softer, elongating the word ("It was h-o-t today!"), taking a very brief pause just before you say the word, and changing up your pitch as you say the word. Again, it's all about choices. Using a mixed bag can make the read much more interesting.