The extra layer of information that you add to a message when speaking is called prosody. The most important conclusion is that prosody lies not only in the voice but also in the facial expression. Further it appears that auditory and visual information together are more effective than the same information separately. 

That a text is more than a series of words becomes clear as soon as you read a story aloud, for example, Little Red Riding Hood. At the end of a sentence you drop your voice. The pitch of your voice also changes when the Wolf speaks. Certain words receive extra emphasis. For example, 'Grandmother, what a great mouth you have!' 'That is to eat you up!'. Not only do you use your voice to make the story frightening but your face adopts a frightening expression as well. When reading aloud you do that deliberately. Unconsciously, you do the same when talking to somebody. 

Czech study subjects 

Barkhuysen used different methods to distinguish the effect of the content and the other information. Czech study subjects were shown video clips with Dutch sentences such as 'God, I feel great', or 'I want to sleep and never wake up again.' Some were shown both the images and the sound and others just the sound or the images. The Czechs could indicate in all cases whether the sentence had a positive or negative meaning. Research with Dutch study subjects revealed, for example, that people can both see and hear when somebody has finished speaking.

Margaret Thatcher 

Research into prosody provides information about the interactions between speakers and listeners. That is relevant for the development of speech computers, such as the NS-reisplanner (Travel planner for the Dutch National Railways). It also explains certain communication problems. For example, some people are interrupted a lot. That is not due to a lack of domination. The former British Prime Minister Margaret Thatcher was also regularly interrupted. The Iron Lady took no notice of this and carried on talking. Yet unconsciously she gave signals that she had finished speaking. One possible solution is to adjust the intonation and only to return eye contact when you have finished speaking.