Talk about what you show, show what you talk about
Too often the speaker creates a narrative that differs from that portrayed on the slide. The speaker's role is to explain the slide. The speaker needs to create a synergy between the oral and visual presentation.
If you portray something on a slide and don't discuss it, you need to ask yourself, "Why it is on the slide?" Remember every unnecessary detail on a slide makes it more difficult for the audience. If you don't discuss it, it probably shouldn't be on the slide. Conversely, if you are going to discuss a point, it should normally be reinforced by a visual on the slide.