In most cases, they should instead slow down. With a book held in front, you can keep your place in the text with a finger or thumb, running it down the page or along the edge. When reading a page in front of you, lower only your eyes, not your whole head. If you're also looking up often, you will hardly seem to be reading at all, and the effect will be akin to true storytelling. Don't be afraid to gesture and move around.
Movement adds interest and holds attention.
And a little miming is fun as well. Try to sound and look and move as you think each character would. Express how the character feels by your voice, face, and gesture. If possible, make each character sound different from other characters and from the narrator. These should be in your normal narrator voice.