Show what you're saying
People process all the elements within an ad simultaneously: visuals, actors' voices, background noise and music. If all the elements are congruent, the brain will process them quickly; alternatively, if all the elements are incongruent, the brain will have difficulty processing them.
To make the ad elements congruent, you need to focus on the ad context, such as having the main character sit in a luxurious armchair (the product) while playing sounds of rustling trees and soothing music in the background. Having these congruent elements in a context makes the brain process information more easily and also improves its memory storage.
An example of incongruent element is a voice-over. The brain reacts differently when a voice-over delivers the message. This auditory information is processed and stored separately from the visual information - an exception to this happens only when the voice-over seamlessly connects to what is being seen.
Take a look at this OHRA advertisement (Dutch insurance company). It's in Dutch, but you can get the gist of the ad. What do you notice?
The voice-over is in sync with the ads visuals - up until and including the scene of the doctor (between 24 and 29 seconds), with the words "...and who isn't" and visuals of empty chairs.
But afterwards, you see the written text of the voice-over saying: "...free choice of care, always, everywhere". However, the visual element of this 'freedom' is not shown. The last remaining visual is that of the empty chairs.
Viewers takeaway the message that they can leave OHRA's health insurance whenever they want, but they don't get the idea of how the free choice looks.
OHRA-ad in the brain
Neurensics studied this ad in the MRI scanner, showing us the specific emotional responses activated by the ad in the brain.
The results showed that although this ad was amusing (humour), it activated little Desire nor Value. Both Desire and Value describes rewarding feelings from using a product, which are important to influence behavior (check out learning 7)
Therefore, be sure to always show what is being said.